At the end of WWII Omas decided to rejuvenate its models and  introduced the piston as a new filing mechanism. Although Lucens and Extra Lucens plunger fillers remained available till the 1950's, they were flanked by a new line of piston fillers which were called Extra and numbered with 55X sequences. 

The new extra piston fillers retained the roller clip typical of the previous Extra models (lever fillers) and were produced in three sizes. Known colors are jet black, brown tortoise, gray pearl, arco brown, arco gray, marbled blue, cappuccino and probably a few others.

In 1948 this line of pens was replaced by the new ogiva-shaped models with the same 55X model name.

Omas Extra piston fillers are therefore pretty rare due to their very limited time of production. 

They are fitted with Omas Extra 585 nibs and are fantastic writers, very robust and with a perfectly balanced size. 

This Omas Extra 556 in cappuccino celluloid is a medium size. Cappuccino celluloid is very often found with discoloration, due to the very light original color. This pen is no exception as it shows a definite brown shade in comparison to the light pearlescent color this celluloid has when unused.

The nib is a very pleasant writer with a great degree of flexibility and springiness.