IIRC, that breechblock is supported by two big lugs that fit into the top of the slide, a bit like the barrel locking lugs on a M1911 type pistol. The welding is just to keep the breechblock in place; the lugs, not the weld, support it against the cartridge pressure.
I checked the front straight "crack" (the line ahead of the weld area) in my pistol and as best I can tell under pretty high magnification, it is not a crack but a mark from the seam of a forging die. The crack in the weld itself is a crack, but of little concern.
Last edited by James K; July 10, 2010 at 10:45 PM.