Early Amateur SSB experimenters built filters from WWII-surplus FT-241 and FT-243 crystals in the 5 MHz range because they were cheap and widely available. This necessitated mixing with a VFO/HFO in the 9 MHz range to access 75M and 20M and, as you noted above, led to the (in)famous sideband inversion scheme. A sideband filter with decent skirt attenuation requires that the individual crystals that make up the filter be matched very closely. FT-241 and FT-243 crystals had the advantage that their internal crystal elements could be removed from the holders and reground if necessary to move them closer to a specific frequency and thus create closely-matched sets of crystals. This is much easier to do with lower-frequency crystals simply because the quartz plates are thicker and easier to work with. There is less chance of overshooting the target frequency by grinding away too much. Plus, surplus crystals were cheap as dirt back then so if you screwed up a few, no big deal. It was also possible to order closely-matched crystals from commercial sources but, hams being who and what we are, this was pretty much cost-prohibitive for the time. Eventually, when commercial 9 MHz crystal filters with decent form factors became available at reasonable prices (e.g., McCoy, KVG), the mixing scheme was reversed: a 9 MHz filter with a 5 MHz VFO. This had the added advantage of increasing stability since it's much easier to build a stable 5 MHz L/C VFO than a 9 MHz one. As receivers were steadily improving (e.g., product detectors and sharp IF filters) and increasing in frequency stability the stability of transmitters became more important. As an aside, anyone who ever used a Drake TR-4 series rig will remember the weird mixing scheme that rig used (a heterodyne arrangement using four crystals) resulted in a sideband inversion on 20M, requiring a special reversed 20M scale on the VFO dial. The sideband selector switch had a "norm" and "opp" position. Why Drake did this, I have no idea, but it worked. The simple solution nowadays would be to simply do everything in USB and be done with it. But, like so many things from bygone eras, the tradition survives. If you can obtain a copy of the ARRL's Single Sideband for the Radio Amateur (an old red-and-black volume) it will contain several articles about homebrew crystal filters. Mine is from 1965. Also, if you can find one, the New Sideband Handbook by Don Stoner, W6TNS, is a great resource. My copy is from 1958. Great insight into the early days of Amateur SSB.