Niklas -- How likely is the "torch job" to compromise the steel in the receiver? If appearance is the only issue, the colors would be easily removed or muted with a little steel wool.
Judge,
Those lockupboxes appear to be case hardened on every M51 I have seen -- many had no case colors left, so is logical guess in some cases. Would presume first damage a torch job would do is alter hardness of outside. Second damage might be slight warping. Given that lockupboxes have plenty of metal to serve as heat sinks, one could wonder just how much dimensional damage has really been done. The few re-case-colored M51 I have seen all seem to open and close normally, but, never made any more than quick, visual check. No idea how case-colors were reestablished. No one I know that has one, has shot it much, so no information to offer there.
I personally do not like case colors and polish lockupboxes on my old guns to give a "French Grey" to Ni-plated sort of appearance. None of them had more than little smidges of badly faded casecolors remaining -- some clearly were Ni-plated at some time in past. One seems to have been faux Frenchgrey.
Those old Husqvarnas are seldom hard to clean up. One needs only be careful to clean up the original "netcheckering" and not turn it into diamond checkering (which they never had), steam out dents, clean up metal finishes, look for old reboring, rust pinholes, bad pitts in bores, etc., replace any missing hammer retaining screws, clean up wood surfaces, stabilize oil-rotted wood, give wood a few coats of TruOil, etc. No big deal at at all. Some will have factory warts, such as firing pin holes not precisely centered on primer, seen on some bottom of line guns (which has not kept them from being heavily used in Sweden. Otherwise, abuse has been from shooter or "restoreer".
Niklas