There have been a few of these I have encountered - all stripped out and destroyed by their owners ....what a waste !
One HW here last year to be put back to 'original' (?) and another elsewhere stripped out and sold to a colleague . Not only .25 but there was also .20 Custom and also the regular .177 and .22 also done allegedly .
I do not think that Mr. Harper signed or stamped his work - most people knew if there was a brass barrel inside then it was HIS work . He advertised regularly throughout the 80's and 90's .
.25 Rhino are the normal brand to use in this cal as the barrel size is unusually tight ...no idea why this may have been ....back then the only off the shelf UK made airgun in .25 was the break barrel Cobra Strike and a custom BSA done by Man Air ....the old Rhino has a smaller head diameter . Or maybe it was simply this barrel size was all that was available to Harper before doing his tricks on it .
In Airgun World an article was written by a HW owner [ Paul Hill ] in issue ...? who gave his airgun to tuner Ken Turner who tuned his airgun after Harper had fitted one of his .25 ' fast spin , shallow groove , rifled brass inserts ' . With only Rhino'S fitting , the max power ran at 11.8 but was reported to feel like firing a 5ft/lb airgun .
Magic