Sometimes, but not always, the promo copies come from earlier in the pressing run with an earlier stamper and earlier in the sequence off a given stamper. But there is no guarantee and no consistency about this. I won't pay a premium for a promo copy, but I'm always willing to give one a comparison listen just in case.
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?