Serial Numbers on Private Purchase Military Rifles

I am trying to assist a member on another forum identify a Mk II rifle

The barrel and forearm are short by approximately four inches and I believe it to be a Mk II* (Mk II rear sight). I am thinking the barrel and forearm were likely shortened. There are indications it might have been a presentation rifle, which suggests private purchase. There is no indication the rifle ever had a rear sling swivel. There are no markings on the right side of the butt and there is no serial number on the left side of the barrel. Holes on the right side of the butt suggest a plaque might have once been attached. The following numbers are stamped on various parts of the rifle:
Trigger guard under side - 2661
Underside of barrel - 3673
Inside rear handguard - 5973
Under root of bolt handle - 1408
Under butt plate - 1456 with a possible "O" prefix or suffix

I am sorry, but I do not have pictures. Think nice looking as-issued Mk II* but with a 24ish" barrel, no bayonet lug or rear sling swivel and no Ross logo, model designation, serial number or year of manufacture.
Do any members know what the various numbers might be? Assembly numbers? Lot numbers?
What is the format for serial numbers on private purchase military rifles and where should those serial numbers be found?
As always, assistance is much appreciated.