I imagine there's only a few corps who make guns for all. Whether they're labeled as faction or civi tech would probably have something to do with private contracts.
Unlawful factions probably have guns smuggled in and then they modify them with black market equipment.
Lib. Transport Turrets: Ageira
Bret. Trans Turrets: BMM
Kus. Trans Turrets: I don't know, Samura and Kishiro are probably still fighting over who makes them better.
Rhein. Trans Turrets: Daumann
Generic Transport Turrets: Different models would have been made by the companies listed above, but the patents may have been sold on to other companies, so that they could be built locally. Their widespread use would allow them to be stolen or the blueprints to be leaked on to the black market, but the original manufacturers probably made enough money not to care about that.
Generic Cap Turrets: Actually a rather silly concept, since there are no generic Capital ships except for the Ahoudori Gunboat... which is really a Kusari ship anyway. For Cruisers and up I've only ever seen these guns used as a cheap alternative for those who couldn't afford Faction turrets anyway. They exist only for balance purposes.
Purple Goddess: ???
Now this is a real oddity in my eyes. The best explanation I can think of is that they're made by a small, independent manufacturer that has no presence in space and is thus relegated to a small handful of factories on a planet somehwere. The most unusual part is that the Purple Goddess gun is only sold on a handful of bases, yet the lower-class variants are commonly available on unlawful and lawful bases throughout Sirius.
I've always seen Debs, Flashpoints, etc, being made by various groups and individuals, using widely available parts. This means that the actual guns themselves might be massively varied, but available just about everywhere. I'd have imagined transport turrets are much the same.
They're not made by any group in particular, but they're put together from similar parts by all sorts of people as and when they're needed.