The 139 measures about 14.1 centimetres capped and is thus distinctly shorter than a 149; the blind cap of the 149, too, is noticeably longer than it ever was on the 139. The barrel of the 139 curves inwards towards the nib, whereas the 149 finishes straight there and has a slight conical step. Early 139s have a flat feed without fins, and the barrel is marked L 139. Because the celluloid layer is thin, parts cannot be turned down into another shape; hybrids arose instead during repairs in the 1950s, when 139 barrels were swapped for 149 ones and 149 caps of that period also fitted a 139 barrel. Such a hybrid can be recognised from the section.
Spare parts for the 139 come practically only from donor pens, since even owners of defective examples are reluctant to part with them. The cheapest route is to buy a second 139 as a donor, for instance a piece made cheaper by an engraving. The piston knob is hard to come by on its own and sought-after parts such as caps virtually impossible; only a very few specialists make individual parts to order. The caps of the 1950s are of celluloid on both the 139 and the 149.