For any prewar BSA all you do is remove the trigger guard and then push the barrel against a piece of carpet as you unscrew the trigger block. Fairly low amount of preload They normally run under 12 ft lb for the best .22's. so the 48 is probably similar. Or less if it's a 177.
I've worked on many vintage rifles. Especially 1906-1930's BSA.
I had to edit my reply.................. I was thinking of a model 58.
First, make sure your screwdrivers fit the screws well! Don't bugger the screws.
Strip it like you would a BSA. Check the leather seal for a loose fixing screw. And anything that might be embedded in the seal. No telling what might have been inserted into the tap and fell through the transfer port. The seal may be in good condition. If it's good do not remove it from the piston. If you do it should be replaced in the exact same orientation. If it gets rotated it might not seal again. If the seal needs replaced a BSA seal from UK might fit. They should be fitted to the bore if you can. I have used springs from BSA Airsporter/Mercury and HW50 springs in the BSA's. Cut them down so there isn't too much preload to reassemble without excessive effort.
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