A bit of research and feedback from a learned member has revealed the symbol to be Danzig. The bolt likely came from a Polish wz.29 that was given to Spain during their civil war (nearly 100,000 wz.29's were supplied to the Spanish Republican forces). The tooling was originally from Danzig and transfered to Warsaw in the early 1920's (Radom factory). It's not uncommon to find a mix of earlier model 98az spare parts on Polish wz.29's. The batch brought into Spain were either thoroughly scrubbed or, more likely, were devoid of identifying marks, save the odd 'z' inside a circle stamp. After the Spanish civil war, many of these rifles were refurbished, which would explain the electropenciled parts.
Mine also has a curved bolt handle characteristic of many wz.29 Calvary carbines. One thing is for certain, it's of a very even, proper hardness and it case coloured beautifully--my Brno 21 needed some bling. The trigger guard and floorplate are also getting colour case hardened. The factory original Brno 21 bolt body is being properly reblued to remove some horrid jewelling, which was the only abomination on an otherwise mint Brno 21.