Do you have any paperwork or pictures proving the connection? I would think that would help.
Also, which bass player? Roger Glover and Glenn Hughes are the two most well-known, but there was also Nick Simper on the first three albums.
I would worry that since none of them are strongly associated with Alembic, that it might not make too much difference in value. Also, by 1978, Deep Purple had already broken up, so it wouldn't be so strongly associated with DP, unless maybe it was Roger Glover and he used it in Rainbow or on his solo album?
Whatever happens, good luck with the sale/trade! Deep Purple is one of my favorite bands and Alembic is one of my favorite basses!