All the ones I have ever seen for sale have never had any maker mark, so I think that we may never know the maker of them.
Maybe someone else could add their knowledge or confirm what I said.
There are two known makers, Deschler and Petz & Lorenz, hower Schickle has been mentioned too.
Here is a quote from WAF moderator Antonio Scapini :
"Both the 8 years are from the same maker, that for some collectors was Deschler, but we found some evidences (from the maker catalog) that might be produced from Schickle.
The 4 years I show is known as a Petz & Lorenz production.
The firms of Petz & Lorenz and Deschler/Schickle are the only I've ever encountered in my researches. I think that this is another field we should study more thoroughly, to avoid some mistakes we have seen in the past."
It is part of an interesting discussion thread
"In all my years as a soldier, I have never seen men fight so hard." - SS Obergruppenfuhrer Wilhelm Bittrich - Arnhem
There are threads about these and the subtle differences between makers here. Try a seach for the thread title and it should bring them up.
Looking for LDO marked EK2s and items relating to U-406.....