That looks like quite a wet line. Do the stamps on the end of the barrel fizzle out, then?
Unusually for a pen I have rebuilt it is not especially wet.
Your question is quite tricky; I have seen leverless pens e.g the L205/47 and L330/64 both of which I have sold (sadly) which had their numbers stamped on the filling knob. The 205 was probably 1933 or thereabouts but I suspect that the 330 was a bit later. I am forever getting Swans that have no numbers stamped on them and constantly have to explain their model types when putting them up for sale. I have made jokes about Mabie Todd's number-stamping man being asleep at the switch. Here's an example:
In theory it's a L470/60, but these usually had hallmarked gold cap bands (I think); mine would appear to be plate. Mabie Todd would do this as well - make a model slightly different in some minor way from the standard: say one cap band instead of two, and confused I presume, found themselves unable to number it!
Sometime in the mid to late 'thirties Mabie Todd started stamping the numbers (when they could be bothered) close to the end of the barrel - Blackbirds incidentally were the same. This continued - erratically - more or less until the end.