Alf
There is not a positive method to determine date of production from worker marks, but one can compare locations of similar worker marks on dated or known contracts. Worker marks and hidden inspector marks are a very misunderstood and under studied part of our hobby, it has only been recently with the internet and digital cameras that we have been able to gather official data.
Worker marks are defined clearly as the letter B through Z but we find much more than these letters on our guns, we still have a lot to learn. For example the circle triangle inspector mark is commonly referred to as the Portuguese Army acceptance stamp but it also appears on 1902 carbines, is this just a coincidence maybe maybe not, the early "shield" barrel inspector mark is also found in the front frame well of guns produced as late as 1912.
I believe that if we categorized worker marks we could eliminate most if not all the fakes currently on the market--how great that would be to reset the clock!!
Sure it would not be long before they learned the trick but imagine being able to buy a 1914 Navy or one of the rare Lugers without fear of getting ripped off!
Worker Marks suffer from years of bad press, I have been refused data on several occasions because it was felt that it was a waste of time. We have a tool to use against the fakers and we are not using it. We are our own worst enemies.
I am rambling on
Vern