There is at least 1 RX-78 unit that exists in cannon that we know nothing about beyond the fact that it exited. That means that there is no reason in the lore that theycouldnt have an RX-78 in the factory
EDIT
To add a little more detail there were 8 RX-78 units produced. Units 1,2 and 3 were sent to side 3 for testing. Unit 1 was destroyed during Zerons attack on side 3. Unit 2 was stolen by Amuro to fend off the attack and unit 3 was taken by White Base to use for spare parts. It eventually was fixed up and turned into the G-3 to test out the magnetic coating. Units 4 and 5 were sent to the Thoroughbred and operated stealth missions in space. Unit 4 was destroyed when its Mega beam launcher overloaded. Unit 5 is still around but it cant be the one here as its armor was different. Unit 6 bacame the Mudrock. It was hevily damaged in a battle against Midnight Fenrir and got retired forom active service. Unit 7 wasnt completed untill 0081 and was assigned to the Phantom Sweep Corps. It is unlikly that this is the one here as its armor was also different. Finally there is Unit 8 that as far as I know has no cannon information available beyond its existence. That is likely the unit sitting in the factory here
