So they're telling me that this is in fact the pulley known as the speed up pulley used with 149's and 1450's and takes a 3/8" belt. Obviously I am still skeptical since it seems like a pretty wide pulley for a 3/8" belt - but obviously I don't know what I am talking about.
I'd attach photos but I keep getting errors, so here's some textCAD...
...ok, well so much for that. Upper PTO input OD 4.25 x .875" and overall height; lower output OD 4.5" and overall height .75".
________<-- 4.25" -->_______ _ _ _v
\ / .875"
/_________________________\ _ _ _
____________|___|___________ _ _ _^_ v
\ / .750"
/________<-- 4.50" --> _______\ _ _ _ _ _^
|