What trans in my Duster?

Hi there, I have a '73 Duster with a 225/1bbl, all original. 3spd auto column shift, just wondering what trans should be in this - 727 or 904? It should be the original, doesn't look like the engine or trans has ever been touched.

Haven't been able to get underneath to look for the 'bulge' on the oil pan as the Year One catalog says.

Thanks for any help.