If the original frame in the pic was a HD or an Outdoorsman, then some welding (or even brazing) was done to the area on the top front edge of the frame/barrel ring.
That to make a wider surface to match up to the Mod 57 bbl's wider rib.
An Outdoorsman frame in that area would have had a flat too narrow to match the wider M57 bbl rib.
An HD would have had a beveled cut as well as no flat at all to start with.
The nickel plate job covers the welding/brazing work.
Welding, brazing,,, any extreme heat applied to the critical parts is something to rethink before firing or putting any further work into it if you have no idea how it was performed originally.
It could be perfectly OK. There are some extremely talented welders,,then there are welders.
If you do reconvert back to 38Special you go back to the narrow rib bbl. You'd be cutting into that area and reshaping it to the original contour.
Just my .02