What are the chances that a double rifle proofed in Belgium, with the lion over PV mark, cal 450 Express, with star over P inspector mark (1952-1960) is a full nitro? The reason I ask is the rifle weighs only 7.5lb and the action appears to be quite slim, no side bolsters.

There is no mark indicating the type of bullet.

According to the above thread, NFB was only used in the transition period between black and nitro which would have been 50 years prior.