Then probably a serial number issued by CAI during import. (the "4/2134") as there are no No4MK2s I can find with numbers like that, most were either "PF" or "UF" followed by a year's last 2 digits, a space then a number starting with an "A".
"UF 55. A189xx"
CAI is terrible about adding new serial numbers, I avoid those. Faz changed from the MkI to the MkII in early 1949. Serial number around PF50XXX, maybe closer to PF60XXX
The six digit PF serial numbers started late 1952 or early 1953?? With the contract series starting at PF301548 and going to PF411471