Had a browse though some old Parker-Hale catalogues. The Miroku over and unders feature more regularly, the side by sides not so much.
![[Linked Image from i.imgur.com]](https://i.imgur.com/9KqI4Fg.jpg)
In the 1970 catalogue they show the Model 500E (BLE) and the 500 (BLNE) is mentioned but not the weights.
![[Linked Image from i.imgur.com]](https://i.imgur.com/oxdXEAp.jpg)
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.