I posted a few pics of this unique old BSA a while ago, with permission of the owner at that time, in an effort to learn more about it. I was only able to offer a limited amount of information based on the owners thoughts, and a brief hands-on session with the gun myself. Sadly, no information as to whom the skilled craftsman may have been who did the modification work, nor the circumstances surrounding its creation have yet come to light.

This unique specimen was recently offered for sale, and it now resides in my collection.
In the process of tearing the gun down to freshen up the obviously dried out lubricants, I had to admire the creativity and skill evident in the modifications carried out.

The rifle began life as a “L” pattern BSA underlever. The original trigger block was replaced with an extension to the main tube which contains a linearly operating hammer, and a hammer tripped sear. The end cover for this extension has a locating pin protruding rearward, which locates into a corresponding pocket in the new trigger block. The hammer engages with a slide unit that travels within a set of rails located in the trigger block. It is this slide unit that is engaged by the trigger sear when cocked. The sliding bar that protrudes from the front of the trigger block is used to cock the hammer, and set the trigger. The bar is operated by the intermediate link of the cocking lever. The whole system operates very smoothly, and the trigger is very crisp and predictable.
The trigger block also incorporates a built in rear peep sight base, and a cross-bolt safety for the trigger.

The gun is very easily taken down by removal of a large bolt located at the underside of the trigger block, forward of the trigger guard. When the bolt is fully removed, the slide that cocks the hammer moves forward, allowing the rear portion of the compression tube to also move forward to disengage its locating pin at the rear, and then up and clear of the slide unit for the hammer. When taken down, the barrel/breech/compression tube assembly is the longest part, approximately 29 inches (73.7 cm) long. Compare this to the length of the OEM barrel on a Webley Service Mk II, at approximately 25.5 inches (64.8 cm). Thus, this BSA Custom is only about 3.5 inches (8.9 cm) longer when taken down.

If anyone can possibly provide any further information with regard to the history of this uniquely modified BSA, I would very much like to hear from them.