The numbers don't always corespond to production date, depends on the manufacuturer...
some Like Robertson and Duralife from the 30's and 40s' put the date on the signs, usually in 4 digit fashion other manufactures have two digit numbers , like on Button signs it will say AM42, or AM 68... those are not the dates rather they are production codes... and still to this date don't have an idea on what they correspond to?
The best way to tell is by determining the logo and what era it was used.. for example the fishtail logo was used starting in 1959 - mid 60's .. that flange is from the early to mid 60's...
also look for any tag lines on the signage to determine age.. like "Sign of Good Taste" late 50's to early 60's
"Pause and Refresh" from the late 40's early 50's
The back of Petrettis Coke book has these logos and phrases used during specific areas and can be very helpfull when looking at a sign.
HOpe this helps.
Thanks for all your input! You are always very helpful!