The back block screws into the cylinder but there is no "cup like" depression as in the Webley Mark3 for instance, so that when reassembling under the pressure of the spring, I found the back of the spring guide kept slipping off the block and preventing the threads from engaging. I cut a piece of plastic waste pipe, 28mm a few inches longer than the block to act as a collar. This collar stops the rear of the spring guide slipping sideways and makes the reassembly much easier singlehanded.

Mike95