Had a browse though some old Parker-Hale catalogues. The Miroku over and unders feature more regularly, the side by sides not so much.
In the 1970 catalogue they show the Model 500E (BLE) and the 500 (BLNE) is mentioned but not the weights.
In the 1976/77 edition there is mention of the “new” MS-150 which is better described in the above 1981 entry where the weight is given as approximately 6 1/2 pounds.
Miroku have always had an excellent reputation for quality.
Post pictures of the British Proof Marks when you gun arrives and we can probably establish the year it was imported.