'50-70 MB (not the 3XX) will be semi-flex or with lots of luck maxi-semi-flex.
My MB 234 1/2 Deluxe is a semi-flex OB, my rolled gold 742 is semi-flex +, in the middle between semi-flex and maxi-semi-flex, my only nib like that. My medium small 146 and so much better balanced than the '70-now Large 146 has a maxi-semi-flex. All are of course stubbed.
So chase the German vintage MB, Pelikan, Soennecken, Kaweco, or Geha pens for the nibs you want.
Only with Osmia/Osmia-Faber-Castel can you tell if you are getting a semi-flex or a maxi-semi-flex. Small diamond on the nib, mostly with the nib size marked in the diamond is semi-flex, large diamond with Supra or just marked Supra is maxi-semi-flex.
The gold nib is as good as the steel ones in Osmia and Geha, in Degussa made them. Great nibs. Osmia not having a office supply house like MB, Soennecken, Pelikan and later Geha was always broke, so had their nib factory taken for debt to Degussa the German gold and silver producer, who continued to make the grand Osmia nibs.
I am using other folks pictures in this in I take horrible pictures.
The first will show the semi-flex nib....I do have a similar pen a 540, made in 1951 just after Faber-Castell took over Osmia.That is a gold plated nib. I don't have one, I have 3 steel and 5 gold nibbed Osmia/O-F-C pens.
The other picture is just to show what a Supra nib looks like.
If you want oblique, vintage German semi/maxi-semi-flex oblique is the only way to fly............I have had oblique nails (no line variation at all), and have still regular flex oblique ...no cigar...the slightest whisper of an echo of line variation.....and modern post '98 are semi-nail....so there will be nothing worth wasting money on.
If you are left handed and not an underwriter.... nail or semi-nail might be nice to write with but you will not get Any line variation!
Semi/maxi-semi-flex are semi-flex, not semi-flex they have a 3 X tine spread if you not guilty of nib abuse. They are flair nibs, not Flex nibs.
In there is ease of tine spread, you get that old fashioned fountain pen script....the first letter of a word will be wider, as will the looping letters and the crossed T's.
If you want a superflex copperplate or Spencerian nib, they are hard to find also in German vintage pens, I have a Soennecken wet noodle, that is better than my two Waterman 52's.
And in real life, a 7 X superflex nib is very rare outside of Utube and folks selling pre-sprung nibs for your convenience.
It is possible to have a 4 X superflex. Most are 5 or 6 X tine spread.
If you want flair, go German '50-70 vintage stubbed semi-flex.
Dip pen will give you want you need for calligraphy..& dirt cheap...if you take and warm up some bees wax put it in the nib, and make rills like this you get a dip pen that will hold lots of ink.
That is lots cheaper than spending big bucks on a passing whim....if with a bit of practice, you find yes you want a wet noodle or even a Easy Full Flex nibbed fountain pen, you can. But I suggest tying dip pen nibs first.....
And while you are at it.........Stiff Italic nib calligraphy is just as great as flexi.
Go to get my hammer and chisel out and hammer that dust rusted shut book open.
Suggest you get one any way, it will teach you how to Draw Letters. Where to push, where to pull.
I found it of much help when I got some superflex nibs to fiddle with......too bad I don't fiddle but when the moon is blue. ...or was that green?