I have a couple of other ESA Dynotron / Swissonic and I acquired a Roamer 612 Searock (MST916 / Steel Case / Blue Dial) a few years ago 'accidentally' in a joblot. Its been sitting in a box for a while as I have other ESA9154 watches so it wasn't sufficiently exciting to finish off. Being a lover of 70s style, steel and blue dials I was kind of thinking of a Roamer 606 Rockshell at some stage. Well I just bought one but it wasn't quite what I expected.....
There seem to be two variants of the both Rockshell 606 Electronic watches. The Rockshell watches without "606" printed on the dial have case number 915.xxxx.606 and those with "606" on the dial have 916.xxxx.606 The 916 versions have the MST916 movement which is an ESA9154 produced from about 1970. My 915 version has an ESA9150 movement in it which was produced by ESA in volume from 1968 until it was superceded by the improved ESA9154.
I've done a little digging and found this Mustang which is a Mustang 601 with 915.xxxx.xxx on the back and an ESA9150 inside - although the movement has not been correctly described on the site in question. But I haven't found any other obvious ESA9150 Roamers. The ad at the bottom of this page Vintage Roamer Watch Site shows a 606 without "606" on the dial in an ad from 1971.
So would it be fair to say that the ESA9150 inside these watches is in fact an MST915? I'm no expert on Roamers but I'm now wondering if these ESA9150 models were produced in the late 60s and then superceded by the MST916 models when the ESA9154 became available around 1970?
Any extra info appreciated as I do like to know what I've got! Photos below so you know what I'm on about - apologies for the quality/reflections - the Rockshells a new arrival and the Searock is a dust-gatherer - both complete, good dials and easily fixable meethinks
Rockshell 606 Electronic / 915.9120.606 / 320.915
Searock 612 Electronic / 916.1120.612 / 191.916
Movements - Roamer ESA9150 on the left with its bridge // Roamer MST916 (aka ESA9154) on the right