That's a "Dana 44" rear axle.
It is THE most common rear axle ever used; it's found in everything from '50s Studebakers to Checker cabs to Jeeps to Broncos to Jags, Corvettes and even Vipers. (variations of, anyway).
You'll have ZERO problem finding a new/used carrier for it, or gears. I have one in my '61, and got a take-out carrier from a Jeep guy off ebay, and bought new 3.42 gears directly from Dana (because I prefer OE factory gears when available).
The early Dana 44s had a more course spline count (I THINK it was 17-spline) and a lot of Jeeps had them. Mine has 30-spline axles, so when you shop, know that there's a difference in carriers.
If your other "spicer" has the same rear cover as the one shown, or has "44" cast right into the housing, it's a Dana 44 also.
If it's got 12 bolts on the rear cover, (but the same general shape), it's a truck 12-bolt.
-Brad
