I found this lens on eBay still new (from old stock) for a reasonable price. This DTL lens is very similar to the respective SX lens, and they share the same design and specs. It is beyond me why Mamiya came out with so many versions of the same focal length.
My assumption is, that Mamiya still offered “regular” M42 screw mount lenses for their TL/DTL series cameras during the time they already sold the MSX/DSX cameras. In order to save costs for production, they tried to use the same lens design whereever possible. The only indication I have for this theory is a price list from July 1978 which showed both, CS lenses and SX lenses.