Shimano makes several freewheels with splined Hyperglide cogs. It just happens that the bore and spline dimensions are different than Hyperglide cassette cogs. As noted in the second post, the diameter of the bore of the larger freewheel cogs has to be larger than cassette cogs.

Shimano's HG37 and HG22 freewheels are one of the great bargains in cycling. Because the cogs are Hyperglide, they shift a lot better than any of the vintage stuff. Going back 50 years, I have tried all of the old stuff, and I can confirm definitely that these new freewheels are better. Yes, they are are of a basic construction, and the cogs are made of softer metal than say the ultra hard steel of an old Dura-Ace freewheel, but they only cost around $20. The old freewheels will shift poorly for a lot longer.

I haven't confirmed that the HG37 and HG22 is still made, but at our Co-op we have a bin of them heavy enough to deliver a strong youngster a hernia. We sell good condition used ones for $5.

EV-MF-HG37 (ax)-1947

EV-MF-HG22 (Y)-1948