First time I've ever heard of that - I must check all eight of mine (Different Mk's) again, maybe over the last 28 years since the mk2 was introduced and I first purchased one, I mist this identification, despite having them apart countless times for cleaning/maintenance over the years
Also the MK2's stock was made from a much cheaper piece of beech, the chequering was machine cut & part finished by hand, the pistol grip & cheekpiece were less defined and my first one - the stock split in two at the pistol grip after only a few months