Seeing as you have the two grooves (dovetails) you can happily dismiss the maxi grip rail issues. Once you have removed the cocking mechanism you unscrew the trigger block, beware it will be under tension which you must counter for the last few threads.
When you reassemble it, the Welsh willy spring being pre cut to a manageable length will be easier to compress as you push and screw in the block. I usually hold the barrel and cylinder upside down on a carpeted floor then push down and turn.
I also try to note where the threads first bite, looking from above,and back up a few degrees which I gauge from practice without the spring.
Give Will a ring if he's got too many emails.
The more I think I know, the more I realise the less I know.