Originally Posted by hokiefyd

Shimano had HG "Hyper Glide and also IG "Interactive Glide". Cogs are thicker on the IG cassettes, but the spacing is the same. If you have a standard Shimano freewheel, nearly any 7-speed cassette should work. It could require a small shim spacer here or there depending on individual fitment.

Or maybe you have something very specific to a certain few years in the 1980s. The Shimano HG pattern is pretty much the standard now, but there was a transition period around about the time your bike was built when even Shimano weren't quite sure what they really wanted to do...