I have the original cracked glass lens that came on one of the four 1952 Panheads imported for the Helsinki Police for patrolling during the Olympic Games. It has the text Harley-Davidson and GUIDE R H 5. Not Guidex.
I'm pretty certain that the lens must be factory-installed. No Harleys were imported to Finland between 1939 and 1972 except these 4 copbikes. As we all know the Tomstone came in 1947. Those were hard times over here and all economic efforts were directed to rebuilding the country and paying the war debts for "attacking" the Soviet Union. In 1952 the debts were paid and some licenses for importing motor vehicles were granted. So it is pretty certain that no pre 1952 spare part lenses came to this part of the world. And even later the poor P.D. gladly substituted the black taillight on my bike for a standard automotive unit, rather than order a new lens.
I bought my bike in 1968 and (amoung other parts) have salvaged two extra Tombstone shells from two chopped "sisters" in 1970. So I do not know from which of these 3 bikes the glass came from originally. But all four have sequential serial numbers and were identical Law Enforcement FS sidecar models.
So it is possible that the 51 used the GUIDE text without "X". It is also possible that for export, the older lenses were cleared from the stock shelves, as the trademark was not so important abroad. Or it may be that black standard units became less popular and assembled leftovers were used up with the earlier lens even a year later.
Theory: Everything is perfectly right, but it does not work.
Practice: It works but you do not know how.
We combined theory and practice: Nothing works and nobody knows why!