I have two, one has your basic cast/drop forged adjusting mechanism, and one has a sheet metal mechanism. The cast has the number 13 on the handle, the sheet metal has the number 47 on it. Are these numbers date codes?

Good question, but I don't know the answer. Hopefully someone will come along soon who does...?

I seem to remember that some one suggested they were mold or die markings. For what reason i think is unknown but the conclusion was they are not year dates.