I've encounterd some complications with the Off Topic rear hub on my On Topic (per KOF rule) 1988 Mercian King of Mercia. It came with 1988 Dura Ace Group, including a 7-speed cassette which is about a 12-19, way too high geared for me. I bought a 13-28 Hyperglide Cassette, assuming these were in use by 1988, but when I went to install it, I found the cassette is pre-HG. It has no lock ring, but rather the outer cog screws on to secure the inner ones. The inner cogs can be rotated to any position, unlike the HG cogs, which have one wider key, so the cog can be positioned in only one way. The outer cog has a different thread diameter than the On Topic AX/EX cassettes, but the inner cogs seem to be interchangeable, although 7-speed of course uses different spacers than 6 speed.
How many different pre-HG cassettes did Shimano make? Do any besides AX and EX fall in the CR era? Did inner cogs on everything before HG interchange, that is can cogs from later Off Topic ones be used as a source to change AX and EX gearing? Does anyone know a source for pre-HG cassettes or individual cogs?