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Does the late 40's early 50's civilian PTO Interchange with the M 37? If not directly, what are the differences?

No.
Straight cut Gears PW vs helical gears M37
Early PW Trans is totally different ,
It is however comparable to some WC's
TGP

It may be possible to switch the idler gear but by the time you source the correct gear for you application and deal with wear and tear items, you might as well start with the correct pto. I know that for the M37, the US pto and the CDN pto are the same, except for the idler gear. The CDN Acme pto idler gear is unobtainium and cost $$$ to have a new one cut for the T98 application. The various other transmission types may be easier to match.

It was a swap meet find and cheap enough to gamble on, so I bought it. We took it apart today and there is no wear or damage to gears or bearings. The shift shaft needs polishing. Case is undamaged. Guess I'll either find a gear or swap meet it out at the January show.

all wcs and PWs have straight gears until 56 when the civilian PW started using the NP420 same as M37. the bolt pattern for trans to bell is different but pto gear in trans the same as M37. So, before 56 only M37 had helical gear PTO and the units for the M37 and late PW are not the same housing and internals as the early stuff, As far as I know the early straight cut gear PTOs don't have any interchangeable gear you can install to make it work in NP 420s

This must have been a late trans in an early truck the fellow dismantled, then. The PTO has a helical gear and cone bearings instead of rollers in the idler gear. I mated it to the early syncro box I'm building for my M37 and it fits and can easily be shimmed for proper backlash.
I did find some wear on the 09195 bearings and 09062 cups, so will be replacing them. Everything else is in good shape and serviceable.
Just one more item obtained for the power module swap project. (Radiator, engine, transmission and PTO as a unit assembly.)