FWIW there are several common manual transmissions that use ATF type fluid (like the T-5 for one), but like you said- sure not a Muncie 4-speed.
The OP could have a synthetic manual trans lube like Castrol BOT 402- it's red and has a GL 4 rating. But it's also thicker than ATF: SAE 75W-85. But at least it would be the right lube.
I'd do the smell test before assuming it's ATF, though. The two (ATF and gear lube) smell markedly different. Hopefully it's been filled w/gear lube and it just looks like ATF.