Shaun, thanks for your input which has just triggered why I was not convinced that all 2446 snap back had the engravings. Most of these unmarked Autavia's seem to come from the later period as per my original post. After a short search I found one on Jaspers site see here:

http://www.classicheuer.de/Englisch/verkaufheuerautavia10engl.html

This is a NOS example showing the concentric circles as you described. Not conclusive but certainly evidence that not all were engraved

regards
Paul
www.heuerworld.com

: Some interesting things I noticed:
: - the 2446 Skipper sold by Arno in the Bonhams auction had no
: autavia engraving on the (perfect) backcase, only a three or
: four small concentric semicircles at the point where you lever
: off the case. I asked Arno about it and he thought it was
: normal.
: - I have seen very good condition 2446 Seafarers with a plain
: backcase and also with an Autavia backcase, both with the
: characteristic "2446 SF" stamped between the lugs. The
: plain one had the same marking at the detachment point
: - I have seen some 2446 Autavia's with a plain back, not so sure on
: whether late or early models. Also with the detachment point
: marking.

: So for me it is unsure. Makes sense that the non-Autavia models had
: a plain backcase, but then sense doesn't always follow in
: vintage watches (seamaster backcase on many speedmasters is the
: classic example).