Having been a Valmet M76 owner since the early 1990s - I know a little about the magazines.
Couldn't find factory Valmet mags, and Galil mags were being sold at the Gunshows as "Works in Valmets". They do lock in, with substantial wobble, but you get nasty jams. I expererienced setback on bullets that would be devastating to a firearm novice that might decide "WTF" why not load it again and pull the trigger. Not at all funny for the Novice!
From experience - there is NO way to modify the Galil mag to work. If the rear lug was ground-on then it was probably being alteres for some other rifle - which makes it even worse.
Then there were the East German AK74 mags that were guaranteed to be converted to feed in all .223 AKs. Bought one for $29.99 and it wouldn't work in S###$t!. Rip off. That was what a Chinese Drums cost back then by the way.
What I finally modified to work in my Valmet were Norinco mags. Dremel tool and a half-hour of gentle grinding and fitting gave me mags that work flawlessly. Norinco mags were not that expensive then. Not sure I would suggest doing it today.
I did run into an advertising error on Sarco's website a few years back that actually showed a picture of a Valmet M71 30 round mag being advertised as "Galil 20 round Mgazine". I kinda snickered, and then ordered the $29.99 mag figuring "What the heck do I have to lose". Happy no matter which one shows up, and if its neither I'll send it back. Got the mag, and it was a Valmet M71 mag. I promptly ordered the remaining two they had. One of the best deals I have made! A little knowledge can really help you out some times.
Never have found a Galil 20 rnd mag. If anyone has a factory 20 round Galil mag - I think I would trade one of those Valmet mags for it by the way!