On the old piston fillers the model designation appears as an imprint on the piston knob at the end of the barrel. The numbers found there, 132, 134, 136, 138 or 139, denote the classic series of the 1930s to the early 1950s. Decades of use often leave the imprint rubbed away and all but invisible under direct light; hold the barrel at an angle to a raking light and it will usually become legible again. The hallmark on the nib, the size of the pen and the design of the cap rings all help to confirm the identification.
Only once the model is settled can a value be put on the pen. The usual approach among collectors is to compare prices actually realised for similar examples in similar condition, with cracks, the working of the piston, the state of the nib and its iridium tip, and completeness all being decisive. Valuations for a fee are certainly on offer, but in many cases they cost more than the pen is worth; they are therefore worth considering only in exceptional cases, for a very rare piece, say, or where a value has to be documented for an insurer.