After some more shameless internet browsing I stumbled upon this thread:

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=133412

10000 circa 1920
15654 8/1927
17012 1929
18914 3/1931
21849 1935 (Added by montenegrin - Febr. 9, 2009)
34874 12/1951
36582 12/1952
40496 1956
48181 1958
59826 1/1964
71829 1969
77667 1971
107321 5/1987

So my "educated guess" is that my Merkel #21849 was probably made in 1933 or 1934.
-Jani

So this one is 18225 - Could it be a gun from 1930? Some of the guys in this thread suggested it was a "pre-war" gun - canny accurate in their guessing if thats the case.

The seller probably reckons 18 is 81 backwards and thus stated " production year 1980".

If its that old, is that a good thing or a bad thing?