What he said...
It does look like 1/2" copper and it does look soldered...
This sounds like it's a temporary thing, so another approach is to affix a 1/2" threaded fitting on the end of pipe...and buy a piece or pieces to convert that to a hose fitting...and when done, just screw on another 3/8" valve like you have to it. Use a few wraps of teflon tape.
Upcoming comedy shows: May 20, Greenwich Village Comedy Club. 6 p.m. See Facebook for other shows and PM me if you can make one...love to see you there!
A recent set: https://youtu.be/995gLmVEVZk
Mangofy your E46!