Really old HG freehubs also had the external UG threading and need tweaking (grinding or a spacer) to fit a cassette that starts with an 11T. Otherwise all 7-speed HG cassettes will fit. You'll probably have difficulty finding tight cassettes, though. Might be easier to just get an 8-speed cassette, sub in all your 7-speed spacers, and drop the largest cog. You could turn a 12-28 8sp into a 12-24 7-sp that way.