I don't think there were anything other than 208's and 241's that are aluminum and passenger drop. If it's aluminum, passenger drop, and not a 208, it's a 241.
Can only find one good pic (link is broken, you can google 241c) that is of the passenger drop 241, it has a small rectangular protrusion on the back side where the drain plug is.
Of course, if it's aluminum cased passenger drop with a VSS, that indicates a 241, period. 1989 only year of 241 with speedo cable.
Looking for the 1989-1991 K5 and burbs being parted out should net you one.
Apparently Dodge may have used passenger drop 241's for a bit, I'm assuming the input shaft and/or bolt pattern is different on those.