Summit still carries M/T Sportsman bias ply tires in L60 & N50 sizes, but not the M50. The tread width on the two tires still available is 10.0 and 11.8 inches. The M50 would have been around 10.9, as an estimate.
BFG makes a radial T/A at 295/50-15. It has 11 inch treadwidth and a 26.7" diameter. The tire requires rim width 8-10" wide.:read2:
I had a buddy that ran M50's on the rear of his Duster, back in the late '80's, and I was wanting to run them on my 1977 Malibu at the same time, but couldn't find them anywhere locally.
I think the closest thing is any of the 325/50-15 sized tires, from at least 4 brands, at Summit Racing, in both bias ply, and radial flavors.