I like to think it's some kind of strange technology from the Moon (so not far off from your Queen Serenity bit). Sailor Moon has a lot of scifi-esque opportunities where Luna and Artemis are using computers and such, and I kind of vaguely tie it in with that. (I also like to think that Artemis also manipulate that same ability, since he was handing items to Minako as Sailor V.) I don't give it too much thought, though! XD Maybe I should!

I always thought that they were stored in the crescent moon mark, and can be called out in a similar fashion as to how some of the senshi's weapons can only be brought out when their transformed. It might be a perk to their cat forms, since we all find out that their cat forms aren't actually their true forms.

It sounds a bit odd thinking about it now, but I always thought Luna just materializes them in same way Usagi materializes the silver crystal, or how pure hearts, dream mirrors, and star seeds materialize out of various people's bodies. While they are always theoretically stored within the people (or cats), they aren't physically stored there or anywhere else, and the only way to access them is through this materialization process (though they cannot always be voluntary materialized and sometimes can be forcibly materialized by others).

With this, I guess you could say there is an alternate dimension (?) or space pocket that purely exists for storing this stuff, which I don't necessarily believe, but is an interesting concept because can you IMAGE the havoc if an enemy found out how to access that place? Also just imagining a room full of dream mirrors, heart crystals, star seeds, and various other magical items just laying there is funny

Well if we go by manga canon the cats are stated to have star seeds similar to sailor crystals so maybe the materialization of the transformation trinkets and other magical items is a part of their powers???

That's an interesting question. I haven't given it a lot of thought to be honest but I do try to dabble at explaining it on my fanfiction somewhat. I'm drawing on what I know from the 90's anime so if I'm wrong feel free to correct me.
The way I see it Luna and Artemis must in some capacity possess some magical power or ability being connected to the Moon Kingdom, that being said in the same way that Usagi and Mamoru can summon items like the Spiral Heart Moon Rod Luna also possesses the ability to summon items, but unlike Usagi Luna can summon specific items at will rather than incidentally. It's possible that Luna may have gotten that ability from Queen Serenity during the Silver Millennium but I like to think it's actually a skill she was taught, but that's based on nothing at all.

I always likened it to Luna or Artemis tapping into some magical 'sub-space' pocket where things are stored.  The magical artifacts that have shown up in the series had to have gone to some place when the Silver Millennium fell.  So perhaps Queen Serenity created this 'sub-space' to store those important things in for when they could be called back again by Luna or Artemis.

Great Idea!!! Maybe that's what the crescent marks allow them to do, like some kind of key to access that subspace though the think of a pocket dimension filled with nothing but Sailor artifacts is amusing

Exactly.  The crescents on their heads act as the key to access these 'sub-space' pockets.  Although, it could be the inherent magic of the two, being they are Lunar/Mau cats as well.  Serenity could have blessed them/given them the power to open these pockets before the final fall of the kingdom, used a little of her own power to essentially put all this stuff into 'storage' so to speak and gave them instructions how to access it in the future.

Then again, it makes you wonder how the Outer Soldiers received their items...

Grr this is making me think too much.  

Well as I said before the manga states their Star Seeds we're similar to those of Sailor Soldiers, maybe materialization/pocket dimension use was part of their natural Powers, which we're modified/enhanced/honed by Queen Serenity, hence the crescent marks.

Well it depends of which canon you go by, but since they only have transformation trinkets in the anime... maybe they came from their talismans??? Or Pluto fetched them in the future and left them for Neptune to find them?? I have no idea, actually.

Yeah, but that's the fun of it, isn't it?

I think the point about these being stored in the same place as the senshi rods/weapons is interesting when you think about that episode where Luna gets ticked off at Usagi for leaving the moon stick lying around irresponsibly.... If there is a subspace then why the need to keep tabs on it? Couldn't they just pop it in and out of "storage"?

I always believed it was some kind of pocket dimension TBH.

Maybe the Sailor Senshi gain the ability to summon their magical items from some magical space as they power themselves up, so they'd have to carry the items manually in their earlier forms, and then later like their transformation items were no longer needed after their upgrades, they learned to summon from inside themselves magically, kind of like with their "Crystal Power" transformations.

For Luna, Queen Serenity could give her some power, so that she could summon it, when she'd need to give the items to Usagi. So, like everyone else said, but kind of like a one-time summoning ability, while the Senshi would learn to summon their items repeatedly.

did I mix up the manga and the anime versions, probably did

I actually always assumed that Luna was *creating* the magical items in the moment instead of summoning them as if they already exist somehow, somewhere. It could be sort of techy Luna is a very advanced 3D printer. But I'm also cool with just going along with the whole IT'S MAGIC~*~*~* assumption too XDD.

In the anime, it seems that Queen Serenity's soul or what have you was also able to give Usagi her Cutie Moon Rod. So I can see Jupi's suggestion that Luna's power is connected to Q. Serenity being likely. Also, in the manga it's shown she can conjure normal objects as well, as she does with a pitri dish in the Black Moon Arc when they get a Black Crystal earring.

As a side note, I wonder if the process fatigues her? 'Cause if so, she would have had to have a HUGE catnap after making all the star pens and communicator watches in R :p.