Henckels' sales catalogs from the 1920s and 1930s are illustrating such Twins trade marks.
A 1929 J.A. Henckels sales catalog is illustrating such one bladed knife pattern with straight handle contour, ref. # 6561 with a minor difference:
the back of the handle is showing up a slightly curved part 'below' the back lock.
Apparently this knife once belonged to a German WW1 soldier, but it's of course like Bernard Levine always says: "buy the knife, not the story".
This is how it currently looks btw.
The pins holding the knife together were worn to a point that the opened knife resembled a baby's rattler, so i'm in the process of making new ones (~2,6 mm) from a similar malleable steel like the old ones were.
The 2,5 mm drill bits just hold the parts together for the pics.