Hi Evert.
I'm pretty sure that the HW55m action will drop straight into a HW99 stock, as should the HW55 action if the stock was relieved for the barrel lock, if that floats your boat ?
The Old School HW50 action definitely fits into a HW99 stock as I've already done it, but the rear stock bolt hole needs a sleeve in it to take up the slack.
I can recall BBS member, Mattyboy, fitting a HW55 action into a HW50 stock, and from a quick measure up this morning the mounting holes on the HW50 and HW55 action are the same distance apart at 7.25 inches.
All the best Mick