"Straight drive" meaning manual transmission? The difference is that the length of the nose which goes into the bellhousing is longer on the MT (and so the part behind the flange that does not go into the bell is shorter). The overall length of the (entire) nose is the same but the flange is located differently (another way of looking at what I described).
I ordered a nice, lifetime guaranteed MT one from Advance and turned in a AT core. Have another (was my only) MT one as a spare.
Note that all of the above applies to small block stuff (assume that is your interest), cannot speak to FE or Lima starters.