[78-L] RCA record prevue

[Francesco Martinelli]

in an Ankara shop I found a bunch of vinyl 78 with indication RCA Victor 
record prevue, DJ copy. I guess they were used for broadcasting since 
they're not in a very good shape and many of them have duration handwritten 
in big letters on the labels. I bought one by the Sauter-Finegan orchestra 
but it's full of crackle. Were these released on vinyl for DJ and on shellac 
for general market, or these late 78 were already on vinyl?
Thanks for any info,
fm