Steve Smolian is correct about the "z" on Victor 78s. The records have
superior surfaces to pressings from previous periods. Much quieter, due to a
different mix in the material used for pressings. Victor seems to have used it from
about 1933 to some time in 1935, after which the material was degraded again
- although not to the level that it had been in the "Orthophonic" period
(pre-1933). Does any member have more specific details?
"Z" pressings are sought-after by collectors for their superior surfaces
and are relatively uncommon because they were issued during the bad times of the
Depression, when record sales were down.