The GWII (Pietta) base pins are double groove as well (hard to tell from the parts drawing).  And I hate Pietta pins anyway, because the groove(s) are only on one side of the pin, not all the way around, so they have to be perfectly aligned when you insert it or it can get jammed.

Not sure if it fits but its $15

https://www.emf-company.com/store/pc/-2-1873-GWII-Cylinder-Base-Pin-p1759.htm

Numrich has the uberti, for $23 … made for the 1873 cattleman BP”
But, having several of those I can attest that the parts are interchangeable with the cartridge 1873 SA

https://www.gunpartscorp.com/products/2042930

And if one needs video halp with a diy here ya go



Yhs
Prof mumbles

Many thanks for all the thoughtful recommendations.  I've checked every recommended source and all were either out of stock or thought their products worth $40 a piece. My inclination at this point is to go ahead and grind one back and go from there.  I figure if I fail, I can always backorder a replacement. 

I totally agree.  I tried to talk the owner out of the bastard-child with the park job, but he was fond of it.

  Lou 

That Rifle be Right Yummy you betcha.  GREAT find