The other thread on this topic seemed to settle that the hole was not for the stock '54/'55 clock.
Here is the
other thread. Here is the location of the
original clock. The quarter size hole through the dash is behind the center of the clock dial.
Of the guesses already made about the purpose of the hole in Steve's dash, I would now guess that it was used for an after market gauge (tachometer, for example) or for a GMC clock which mounted against the dash without the Chevy-style clock housing.
I am interested in hearing if Mongo's clock housing fits a '54/'55 Chevy truck dash.
Tim