The two block letter brand marks are the earliest, probably 1920's and
early 1930's. I don't know if they were used simultaneously or one before
the other. Apparently the earliest production in the teens was not branded
as I have heard.
The next oldest is probably the straight script with the M with pointed
tops. I have seen evidence of this being mid 1930's and likely to about 1940.
The newest marks are the arched ones and especially with the trademark R
The straight script with rounded top M appears to run from roughly early
1940's (maybe late 1930's) to near the end of production in the mid and
There is a lot of missing info of this type. I have just made guesses
from various bits of information and clues so don't take my opinions as
Any info or comments anyone has regarding this would be welcome.