I was reading Dominique's blog this morning and saw this nice Arc de Triomphe cover here. Now I collect this set, too, and the number on the sheet margin has baffled me for some time. I have a block of 50c stamps from the bicoloured issue, and that has the number 45801 (I think).
Dominique calls it the plate number. When I was last in Paris I went to one of those wonderful philatelic shops and the lady there, too, said it was the plate number. Now, the stamps are printed in lithography which means that it is quite normal to have a large number of plates used as they apparently tend to wear quickly. Here in Britain a point in case are the 1990s machin booklets. But when I say large, I mean dozens. Not 45,444 different plates as in Dominique's copy or 45,801 as in mine.
So what is going on here? Maybe it's a case of just one digit in the row of digits changing per plate. So my next plate for the 50c could be 55901. Or maybe it just isn't a plate number at all. Maybe it's a sheet number. Again, here in Britain each sheet has its own number, and with thousands of sheets printed, a large number is very common and logical. But it could also be a batch number or maybe a production reference number, or whatever. The stamps were printed in the USA so it might well be that this sort of number was used on sheets of American stamps too. We're talking 1944/1945 here, by the way.
I've sent Dominique an e-mail regarding this, but I thought I'd put it up here as well, because maybe one of you will know!
And while I'm typing this, I've received an e-mail from Dominique, pointing me to another example shown on this website. The 1f50 has the same number as on Dominique's cover. Dominique also makes a valid point that a sheet number would probably not be in the stamp colour, but more like all the other text shown in my image. That does point to a plate number. Maybe plate numbers don't start with 0001 for each stamp, maybe this is just plate no. 45444 that the printers produced in their stamp printing lifetime.
Anyway, any more info is welcome!