Colt started the sear block thing by blind-pinning in a steel block where the auto-sear would normally be installed in an M16 lower. This made it very difficult for people to add autosears, whereas before, they could have just drilled a hole in the lower and installed one (illegally for the most part).
But these days, no one uses a seperate block. Now, all AR manufacturers leave some portion of the lower receiver unmachined to act as a sear block.
Colt, Bushmaster, Armalite, DPMS and others leave one of the sidewalls a bit thicker in that area, which would have to be machined out before an autosear could be installed.
PWA, ASA, and Rock River leave the "shelf" area behind the selector/safety about 1/8" higher than normal, again to require machining of the lower to install an autosear.
None of these blocks are legally required, and it is legal to remove the blocks. It is NOT legal to drill an autosear hole or install a sear without the proper license and/or tax stamp from BATF.
-Troy