All fountain pens with the new half covered nib have now no screw cap, but feature a plug cap. They all feature a revolving window that shows the level of ink. As a further feature every fountain pen has a wide metal ring at the transition from the grip section to the ink window.
Rules of the model name: The model number starts with Mxx or MKxx. The M stands for filler with mechanics (ie piston filler). The models Mxx have a metal cap, either made of nickel silver (Silvexa) or gilt nickel silver (gold Doublé). The model numbers MKxx denote the less expensive models with plastic cap (K = plastic in german "Kunststoff").
In two exceptional cases, these rules differ: the Model MK10 black / green is as identical model called 120 black / green. The model MK30 is black also known as identical model under the name M25 black.