The reason I said either slicer or ssb adapter is as follows:

A slicer allows one to copy and select the sideband
on an older receiver such as the Hammarlund 129X.
The most prominent slicer was amde by Central Electronics there were two
versions, one just a slicer and another with a Q Multiplier which was
usually missing from older receivers. There were several other slicers on
the market. I had one called a VR
made in New England. RME made one to match the 4300/4350 receivers. This
was the only one that did not match 455 kcs IF's.

The best example of the ssb adapter was the Hammarlund HC-10 which added
another IF plus a slot and pass band filter. The SPC-10 was exactly the
same except sold to the DOD for six times as much as the HC-10. Frank
Lester (SK) designed these and helped incorporate them into several of the
Hammarlund receivers. TMC and several others made ssb adapters, too.

Both the slicer and SSB adapters made copying CW better too.

Dave K4JRB and still looking!