It might be someone's project. Or, many years ago, No. 5 rifles were worth somewhat more than No. 4s, and a company in the US bubba'd up reworked No. 4s with full length barrels fitted with muzzle cones. Golden State Arms in California? Yours could be one of these.
If it shoots well, it should be a good rifle.
Incidentally, during the 1920s, nomenclature of British rifles changed. Primary models were given a Number; variations a Mark designation.
So, a SMLE became a Rifle No. 1 Mark III (or III*). The later Lee Enfield was the No. 4 Mk. I (or Mk. I*, I/2, I/3, II or 2, depending). You were hoping your rifle was a No. 5 Mk. I, not a Mark 5.