Ken,
Apparently you have a "Sea Rock" or "Old Sea Rock" that is an early pipe made for the American market that included the red diamond so as not to be confused with a Wally Frank line made in the 50's that has a white bar similar to the Castello's sold on the European market.
This is according to Robert C. Hamlin who wrote the Castello Grade and Style Guide for the PCCA.
American logo'd Castello pipes use a small round "Diamond"
(referred to and looking like, but it is NOT actually a diamond)
inlaid into the mouthpiece. This was originally done so that the
standard Castello white bar logo did not conflict with another
brand and logo that was sold by Wally Frank called the "White Bar
Pipe" (in the 1950's).
Many Castello collectors prefer the white bar logo and a few
prefer the "diamond", however there is no real difference between
the two once you compare like pieces. In a few cases there are
differences in finishes or grading; we will point these out when
they apply.
SEA ROCK [Carved Black or dark brown]: This is the lowest grade
of the Castello line and is the most common in the USA. Sea
Rocks are produced by taking a smooth bowl that has not been
"final finished" and surface carving the finish with tools. This
"carved" finish is then evened out using a steel wire brush,
stained and then waxed. The Natural Vergin carved finish is left
unstained and unwaxed as a rule, although we have seen waxed and
partially waxed"Vergins".
http://www.pipes.org/FORMATTED/196.html
According to Ed Anderson this red diamond could be on some pipes produced up through the 70's as well. Either way it must be a some what early Castello.
http://forum.pipes.org/~discus/discus/messages/26/33399.html?1154303560