Answering my own question here, but maybe this will be useful to somebody else
in the future (or present?).
US patent 2,607,699 granted to Audio Devices Inc. (makers of Audiodisc blanks)
lists the following formula:
33.3% 1/2 sec. nitrocellulose (1/2 sec. is a measure of viscosity)
18.0% wetting alcohol (ethanol or butanol - solvent)
28.2% ethyl acetate (solvent)
10.0% dibutyl phthalate (plasticizer)
10.0% castor oil (plasticizer)
This formula must have varied between brands and lines, but it's a step in the
right direction for me. I'll update again if I make any windfall discoveries,
but until then I welcome any further info!