As Mike Kone says, the 109 series are either very early bikes (http://www.r eneherse.com/1945racer.html for example), or bikes that have been back into the Herse workshop for restoration, light or extensive, or indeed frames f rom other makers that have been turned into Herses.
This last occurence is quite rare, but I do know of one frame that quite de finitely wasn't made by Herse (it's awful...), with a 109 number and Herse paint, and another complete bike still in the hands of the original owner, made by another maker, that he took to the Herse workshop to be fully "rest ored" as a Herse. It has RH brakes, a lot of braze-ons added in Herse style , but the basic frame underneath is so obviously not an Herse, that the own er's story is definitely confirmed.
So to my eyes there is no doubt that Herse took in bikes from other makers, as Singer did, and turned them into his own brand. It's extremely rare, bu t it did happen.
I will try to get authorisation to use the pics of the reworked bike mentio nned above on my site, it is a bike that is historically interesting, and t he end result is a nice bike, albeit a bitza.... I don't know it's serial n umber, but it will be a 109 for sure.
For the later bikes the number system is straight forward, 10 56 for exampl e is the 10th bike made in 1956.
Dating the 109 series has to rely at the moment on personal experience and judgement.
Alexander March Bordeaux France