That is a nice EF.
I chase the nib...or try too. Oddly, I'd not kept track in my mind that the following two were two toned nibs. .
Pens gotten later in one's collection; or selection might be a better word, don't stand out quite as much as earlier pens. I was forgetful when young too, so don't worry about it.
I have a 70s-80 model Large 146 that I judge a B (in there is no marking), , monotone gold 14 K regular flex. ( I'd swore it narrower until I just tested against my KOB.) I don't know if MB was making wider nibs then than in the '50's. If so it could have been sold as an M.
The 'springy' modern 2000's nibs are wide.
And I finally lucked out getting a medium-large '50-60's 146 maxi-semi-flex, with a flatter, faster feed, which is two toned 14 K. OBB.
I just ran a test of it vs a monotone 14 K marked semi-flex 234 1/2 Deluxe (52-54 only) KOB. Nibs were 1/2 a width narrower back then. An OB could be mistaken for an modern M.
My rolled gold 742 (51-55) also has two toned 14 K nib....matched vs the KOB I'd say an M....but with out that I'd call it an F. M-F??? (Between a semi-flex and a maxi.)
Comparing it to yours, I'm sliding more towards M..........I really don't worry much about exact width...in with slop/tolerance and different era's and other companies having the same problem with nibs in their standards ...one compares apples and pineapples.
I prefer the balance of the older middle-large 146. That one I can grab and never have a second thought. The newer Large 146, I always need that minute to get use to it being a Large pen.
I'm done hunting MB's.....a very slow hunt, but the three flex's of the '50's pens is what I was after....actually just a semi&maxi....but will take the middle too. (I have other semi&maxi's by other German companies. 2 of 7 Pelikans 1 of 5 Geha's, and 1 of three MB's.....I don't count Osmia/O-F-C, in they labeled their nibs, Diamond for semi-flex and Supra for maxi. So one knows what one is buying.
Counting the regular flex and a modern 'springy' fatter nib, I've got what MB offers nibwise. Well, I did test a Weak Kneed Wet Noodle '20's Safety pen....that went in a live auction for much more than I could think of.
In I refuse to practice...two wet noodles is enough (from other companies)...and '30's MB's are over my limit.