Do you really think that PH would have sent the CF an un-serialized completed receiver in 1990? It wasn't 1945 or 1955 any more -
00341G could easily have been retained by PH as a sample, or failed QC and been rejected by the CF - or it's possible that PH built 2 rifles with the same serial number.
Serial numbers are serial numbers - they are assigned by the manufacturer, the CanadianGov't doesn't own numbers or prefixes or suffixes- as an example, FN built many Serial #1 FAL rifles and HP pistols - Beretta and Breda built many serial # 1 Garand
rifles.