I think the experimental sides were pressed on what came to be called Victrolac when Victor used it when they took their abortive fling at lp's around this time. The processing of the Bell discs was supposedly very carefully done: they used gold in the first plating bath for example. Bell was working with Stokowski at this time as well. He was always hot on the trail of better sound and the Philadelphia orchestra was recorded in real stereo in a performance Scriabin's
Poem of Ecstasy which I think was issued by Bell around 1980 when they did a limited edition of some the 30's stereo experiments; which is where in fact the Heifetz may originate. I've never seen the lp.
Bell had made significant advances in microphone design at this time and it certainly shows.
Speaking of processing, I understand that much of the high end was lost in the processing and pressing of a 78. Charles O'Connell , Victor's A&R man in the 30's goes on about this in his book
The Other Side of the Record. He is sour about the loss of quality in the plant, but then he's sour about MOST stuff in that book...
Decca in the UK when over a lot of this same experimental territory when they introduced their
ffrr recordings in the 40's. They found that the processing was where a major portion of the high end vanished.
Jim