Thanks Ron.
A minor clarification:
These tiny, smudged cancellations, which are typically applied in almost the same ink color as the stamp, are
very challenging to identify.
In the case of the stamp shown above, it is actually cancelled by B W & Co., with the "B" portion being barely inked.
See detail images below.
Jim
M. & Co. (Merck and Company, Incorporated) looks like this:

B. W. & Co. (Burroughs Wellcome and Company) looks like this:

Note that this example is a particularly clear cancel; usually they're waaay more smudged.