I used regular paint and varnish stripper (not the one that removes stain), sprayed it on, when the varnish bubbled up and lifted clear of the wood surface and I just brushed it off. No effect on the wood or wood finish, grain, etc. I don't leave it on there longer than about 15 minutes. Never use any abrasive. Then, wipe it with acetone quickly. Never had a problem with blue removal in many years of this method. It simply reveals that which was there when the varnish coat was put on. Sometimes it is like this, sometimes it reveals sanding and prep work.
Craig, Looks like a Ruhrstahl barrel, - seems like bys and a blurry pattern that looks like Ruhrstahl? Is the barrel e/13 marked?
Gustloff/1942 was their most interesting year for BC as they used some rather unique makers that year, some only known in 1941-1942, unfortunately most are Ruhrstahl, but you can find the Dutch AI (guo) this year. Let me try and guess avk 122 41 bys e/13
If you know already... no problem if not.