Perhaps not, but the tube guru at work claimed that batch's of 12AX&'s that had low gain were commonly labeled as 5751's some years ago, despite not being of the mechanical construction, or having low noise or a lack of microphonics.
The manufacturer's were aware of the buzz back then, and unscrupulous tube dealers could easily pick up on the current 5751 swell. Prices have already gone up on this model, as I was perusign some for my Aronov. Alas, I blew an output tube the other night, and will have to re-tube before I can proceed further.