Thanks Zaf - fascinating ..
Was thinking of the scan at http://zegarkiclub.pl/forum/topic/12856 ... C4%85cych/
and the examples cited over on purists - with very few exceptions, what folks call an E159 seems to have a snap back ().
I seem to recall someone dug up that they can crop up with screw backs as per viewtopic.php?t=450&start=10
(and I think Ilja had one?) - but those are few and far between (and for issued, like Hens' teeth..).
.. does that suggest that properly, it should be called a 159 unless its got the screwback, when it becomes an E159
.. mine looks like this for what that's worth:
 - the SHOM riff on this pattern all had snapbacks .. ?