Most definitely a pin. Been told very very hard to remove. Item W in the image. Need to get the torque tube off the housing first.
1940 and later had the torque tube flanged to the diff housing. A lot simpler than the riveted 1939 and earlier design.
The other image is a 1939 46C with...
Depending on the year of your car, will also need significant engineering changes from torque tube to open drive for the diff location. Has been done before, but a lot more than just changing the transmission.
Most (all?) of the (easy) swaps are into series 40 Special & series 60 Century cars using diff's up to 1954.
I do not have my parts books with me, but from memory series 80(70?) Roadmaster & series 90 Limited diff's are very different. Brakes are a lot bigger. So I suspect not the simple swap...
Not for 1946. And no "build sheets"
VIN did not start until mid 1950's and were not standardised until 1981, with the 17 characters
https://en.wikipedia.org/wiki/Vehicle_identification_number
In USA some states titled cars by the engine serial number and others by the frame/chassis serial...
Nice car. What are your intentions with it?
4557 = 56S Sports coupe with rumble seat
4 = Buick
5 = 50 series
57 = body style
GM Buick Canada had a different numbering system to GM Buick USA
The topic may get more traction in
https://www.teambuick.com/community/index.php?forums/transmissions-and-drivelines.79/
There a few suttle difference between the Owen C recon torque ball ex Napier (as photos above) and some photos I have of a NOS
From
http://www.classiccardatabase.com/search.php?year=1951&make=Buick&series=4843
1951 46 wheelbase = 121.5
http://www.classiccardatabase.com/search.php?year=1975&make=Buick&series=7773
1975 Rivera wheelbase - 127
But I have read about guys putting old bodies on 1970's chassis. You just...