Is it a #1 MKIII or a #1 MKIII 2A? the first one is 303Brit, the second one is .308, there is a big difference. To tell the difference the easy way, is to look at RT side of the reciever, below the bolt handle. if it has Maltby, Fazerlakey, BSA or Lithgrow it is a .303 Brit Caliber Rifle. If it has "RFI" and a date, it is a .308 and was made at the Ishapore Armory in India. Or take it to a Gunsmith for a positive ID.