Original Mod 24
Thanks for the information. I cannot clearly make out the year of manufacture, it could be 58, 68 or 88. The cylinder has very short and shallow scope grooves, and the sights on the barrel are non-adjustable, apart from by drifting sideways. I have a recollection that the seller told me that it was made up from "bits and pieces", which didn't bother me. I've had a look at the 1980s Model 24, and the cylinder doesn't look the same as mine. Maybe it will remain as one of life's mysteries, and be hailed as a unique model in a few years time!
Life is to be enjoyed, not endured.