In looking at lots of original Spencers over the years, I have noticed that a large number of them have a portion of the left chamber wall missing where the chamber wall is thinned for the blade extractor slot. The missing portion is a few thousands of an inch thick, about 3/16ths of an inch long, and all the ones I have seen were roughly triangular. By my rough estimate, about half to two-thirds of the Spencers seem to have this defect.
I would guess that because the missing metal is so thin, and the cartridge cases are so thick at this point (at least for those using modern cases), it shouldn't be a problem, because the missing metal is pretty minimal. Also, the extractor supports the cartridge case at that point. However, if you were shooting a Spencer that did not have an extractor in place, I could imagine some problems that could crop up--shortened case life, stuck cases, or even case failures.
Has anyone else noticed this? How common is it? And more importantly, has it ever presented a problem for anyone?
#1692—M1860 Rifle with intact chamber
M1860 Rifle--closeup of thin chamber wall over extractor slot
1707—M1860 Rifle with chipped chamber, showing extractor
M1860 Rifle--closeup of chipped out section between chamber and extractor slot.
DJ