I have seen Byrdlands with Bigsbys , and of course with the usual tailpiece ,... but this is different.
1961-1963 on SG Custom and Standard stock. The end of 1962 saw the introduction of the lyre engraved Maestro that we are all familiar with. A few '62s and most '63s have the Maestro, but '63s with the side to side are not uncommon. Looks great on the Byrdland!!! I don't doubt that it is stock. To verify that take it off and see if there are any other holes.Apparently 1962 was the first year these tailpieces were used.