The pen will be about 136-7 mm if it's an oversize 557.
When Omas first came out with piston fillers, just after the war, they had ink-view windows. The celluloid of the window was different from the barrel and over time tended to shrink. This meant that the piston seal had to negotiate different diameters along its travel, not good for maintaining a tight seal.

(Discussed in a previous
post)
Soon afterwards, around 1948, Omas redesigned its piston fillers along the lines of what you now see in your pen. Gone was the ink-window, the roller clip and the triple capbands.
The first models had a barrel imprint of Omas Extra in a cursive-like typeface.
Click to view attachmentThey eventually changed to stylized block characters as seen in yours.
Click to view attachmentThere is a variation between these imprints but I'm not sure whether it was only for the round 557 S model or also for the faceted 557 F as well.
Click to view attachmentAs you can see from the above pics, these pens initially came in more colors. The tortoise version above was one that I bought but which the seller broke while he was preparing it for mailing. By the 1960s or so, the color variations were reduced to Grey Pearl and Black.