Unfortunately, it's a myth that Osmia produced the Merlin pens and this myth was spread erroneously by Andreas Lambrou. Yes, there are similarities but the build quality is not the same. But many similarities of German pens from that period originate from the fact that many parts were bought from small producers and used in various brand pens with minor modifications.
A while ago I bought a bunch of Merlins from someone who knew a bit more about the origin. Maybe the pens were produced by a number of tiny manufacturers who made the barrels. One of them was the company Huch in Gross Bieberau (near Darmstadt) where also Senator has it's headquarters. The founder of the company now called Senator, originally called Merz & Krell, was Friedrich Merz, a pharmacist, together with his brother Georg and an lathe machinist, Justus Krell, who is said to have learned his craft at Osmia. The lathe shop in an old mill was taken over by Huch later and produced the barrels for the Merlin pens. All parts are pretty much standard and appear to be from unknown small manufacturers which produced for all the big brands. This would also explain why for formally the same model one can find various nib and feed designs and even clips. It's not like a Pelikan, Montblanc, Osmia (Faber-Castell), Soennecken, or Kaweco of that time to name just a few quality brands who produced consistent lines of pens.
According to the seller, this information is based on statements from a co-worker (actually a collector) from Senator and Beate Huch, the head of the Huch company. I can't verify them but based on my own experience with Merlin pens this appears to be very reasonable.
You can help spread the word.
P.s.: The pharmaceutical/chemical company Merz was also founded by Friedrich Merz.