Most of the WLPs I have are CBS (Columbia/Epic). The ones from the late 50s and early 60s are pressed on quieter vinyl than the standard issues. Same thing with the Mercury promos I have, except the promo labels on these are either gold or sort of peach-colored. I have a few 70s CBS promos too, which have white labels, but don't seem to be of any better quality than the regular issues.
Promotional copy - any different in sound quality?
I wonder whether promotional LP copies "not for sale" were made any different leading to better sound quality. Are they any more worthy than non-promos?