Looking at the chambers diagram...
https://www.gunspares.co.uk/products/24567/HW57/
...the rear end looks to be the same set up as a HW95. The caps can be pushed out from the inside with an allen key (sometimes you have to grind down the short end to fit). Once you have the caps on one side out, you can punch straight through to get to the other side. Once they are out you need to take the preload stress off the trigger housing, you can use a pound coin or similar wrapped in tape to do this: just put the coin on the floor, push the housing on to it and give it a small twist, and then carefully release the tension.
This is a much better explanation of the above:
http://www.freewebs.com/stripdowns/hw9895kstrip.htm
The one thing about that guide is that he doesn't put anything over the coin, so you could scratch the housing. Bit of tape will sort that.
I have no idea about the front end of the '57 though.