That's  what I hoped it was..  ( being  built  to accommodate  the larger pinion in the first place).         

Do you remember if that Gas starter looked like this one, or more like the  direct drive  delco? 

I did the math,   the  gas/propane       used  an  18.33:1  ratio     9 tooth/165 tooth

                                diesel    is          14.2:1  ratio.     10 tooth/142 tooth  

I suppose cranking speed was the determining  factor?