You could send either rifle to Martin at https://www.airguns-usa.com/ and have him do any ole thing you like. Reg the HM and have it set bup for "mostly 20 yard" shooting and you should have a nearly motionless rig that shoots all day on a fill. Now that .25 is old obsolete and just does not sound right for you so I'll trade ya an old cheap .177 that's only been tied to the rear truck bumper a few times (but it wasnt raining) that shoots nicely at 20 yards. Or not.
The Hm1000 serial number number is 0166 and the Theoben is 0098. But there is no online record of when they were built. I was able to find plenty of build date info before they moved production to the USA but none since. Here are a couple quick pics of the Theoben.