The "67D" number was the Atlas "tail number". It showed that this was
the 67th D-series Atlas booster built - one of several Atlas-D boosters
that were built on Convair's ICBM production line for the Mercury Atlas
program. Other D-series Atlas vehicles, which had radio-inertial
guidance, were used for ICBM test launches, served as operational
missiles on standby, or were topped by Agena upper stages for unmanned
space missions.
- Ed Kyle
http://www.geocities.com/launchreport/