McMillan's Lazzeroni stocks, the thumbhole and sporter stock, were made for John Lazzeroni's rifles by Randy Lawson of Harry Lawson Co here in Tucson. Lawson's makes wooden versions of the thumbhole which is the same shape but comes with a recoil absorbing flared recoil pad and a very deep durable finish in high gloss or satin. They also offer a crossbolt safety which works well with the thumbhole design. They have been making this thumbholes since the 60s.
Here is a left hand example