Great collection Flint. You have done extremely well to put together a collection of Haenel 28's like that in 18 months. It took me more than twice that length of time to get the same, and that was 25 years go when there were a lot more collectables about. The .22 repeater is especially rare in the UK.
Incidentally I have never come across that locking screw before.
You say that your Haenel collection is now complete, but I bet you anything that you won't stop there. Have you thought about adding the "Haenel Air Pistol" to your collection? - basically it is a very early Haenel 28 which was sold before Haenel decided to add the suffix "28" to this design to differentiate it from the
lesser Haenel 26. They are simply stamped "Haenel Air Pistol" . They have a flat grip emblem, without the usual outer rim, and the cylinder lock-up unit uses a notched peg rather than a hook. An example of this pistol is shown in the pic below.
Then are the two Haenel 26 variants, pictured with their big brother, to look out for. Probably not your cup of tea due to their mainly alloy construction and lack of walnut grips, but even so they are an important part of the Haenel story. They would certainly put the lid on your collection.
Happy collecting for 2018,
John