Thanks for all the responses!
My first preference is definitely 56-50, but I have a nagging fear that I may not be able to get one in Canada. Some of you may recall my abortive attempt a few years ago to purchase a 56-50 carbine from our host here in Cas City, Marshal Halloway. He got me a mould, brass and loading dies, and tried shipping ithe whole package to me twice, after jumping through hoops to get the necessary paperwork in order. Both times, it was bounced back to him by U.S. authorities, due to the restrictions on exporting firearms of .50 caliber and larger ..... an odd little restriction enforced by the Dept. of State (not BATFE). Ultimately, he could not get this export exempted from the restriction (which is really aimed at the stuff chambered for .50BMG and such, of course) and ended up having to refund my money.
I could soon be "up and running" with ammunition for a 56-50 Spencer, because I consoled myself by spending the refund from Marshal H on the original Peabody with Canada Militia mark mentioned in my first post. It is chambered for the very similar Peabody .50-60 rimfire cartridge, and I have it fitted with a one of the 'New Old Stock" breechblocks Dixie used to sell, converted to centerfire. I load shortened .50-70 brass in a set of .50-70 dies obtained expressly for it, but do not shoot it a lot. If I can get a 56-50 Spencer, I would simply have to get a separate batch of brass, as I understand that the Spencer cartridge is a bit shorter. (Interestingly, records show that when Canada had both Spencer and Peabody rifles on issue, they issued only Spencer cartridges, which apparently functioned fine in the Peabody .... albeit with less than stellar accuracy, due to excessive 'bullet jump', apparently.