STC schematics are also stupidly rare, hence the lengths the AdMech go to in order to get them.since the 41st millennium is known for using STC (standard template constructs) for, like, everything, i’m not sure if that’s entirely true. unless it was made by Xenos races?
I'm sure I've read a 40k novel where they talked about finding the ancient STC machine for what became the standard issue IG knife.
But people still had knives before they found it and continued to have non-STC ones afterwards - the same can be said for almost any 40k tech I should think.
The munitorium "marks" or "patterns" are probably STC produced, but there will be non-STC analogues all over the place. Some might be better but expensive, some worse but cheap, some better and cheaper but not blessed by the omnissiah so if you're caught with it and don't happen to be an inquisitor you'll probably find yourself exterminated...
Almost everything produced by the Adeptus Mechanicus (i.e. all sanctioned tech) is based on STC designs as far as I know. The AdMech sees the STCs as the pinnacle of human achievement, and trying to invent beyond this is the height of hubris.
Naturally, there's variation. Both the standard Cadian-pattern M36 lasgun and the similar, but it and the subtly different Catachan lasgun are probably based on an even older lasgun STC pattern. Flak armour, at least the material itself, is probably an STC. The Chimera chassi is an ancinet STC design (The Leman Russ isn't, IIRC, as it was invented during the crusade which is notably AFTER the Dark Age of Technology).
"Modern" plasma guns are based on an ancient, pre-heresy design originally held by the forge world Ryza. However, according to old fluff, they borked up copying the blueprints over the ages and introduced an error in the cooling system. Thus, modern plasma guns have "Gets Hot", which can be removed by manually tuning each weapon (in the RPGs represented by better quality). This blueprint eventually proliferated to all other forge worlds producing plasma weaponry, so Ryza is the reason why your Space Marines sometimes melt their own faces.
Of course, local gunsmiths can and do make variations on tech, and the less advanced it is the less likely it'll be that the Red Priests will take offence.
Make knives slightly longer than standard? Sure. Slap two toasters together for a four-slice toaster? Whatever. Slap flak armour plates on a tent to make a shrapnel shelter? Sounds good. Scaling up autopistol from 9mm to .45? Probably OK. Change furniture on a lasgun? Who cares. Convert a 12ga shotgun into firing bolt rounds? ...Now that might be heresy.
I'd like to think of the AdMech proper as both the big industry concerns and the standards organisation in one. They make most of the stuff and set the standards, and if you want to legally fiddle with tech, you'll have to have a permit from them.
Not that any self-respecting underhive gunsmith ever even saw one of the red-robed cogheads, and if they did, they'd loot their stuff.
I thought the Leman Russ was an STC that was discovered during the crusade?(The Leman Russ isn't, IIRC, as it was invented during the crusade which is notably AFTER the Dark Age of Technology).