In many years of collecting, I have rarely encountered monocoque models powered by calibre 551 movements. Monocoques are two-piece cases that do not have a removable case back, and where the movement is accessed from the front of the case by removing the crystal. While calibre 564 movements in cases 168.015 (domed dial) and 168.025 (pie pan) regularly surface, the non-calendar models 167.015 (flat dial) and 167.025 (pie pan) are hardly seen at all.
The 167.015 shown in the picture above and below is the first example of a flat dialled non-calendar monocoque I have encountered in more than eighteen months. I could count the number of pie pan dialled 167.025 models that I have seen over my entire span of collecting to no more than a handful.
The model pictured does not appear in the Omega database, and nor did models 168.015 and 168. 025 until I made representations to the Omega museum. This may account for the lack of interest in this watch, recently sold on eBay for the incredibly low price of around USD 320.00!
There are a number of reasons why global inventory of the 167.015 and 167.025 is small. The first reason is that they were produced at a time when calendar watches were a fad in full swing. People generally eschewed the non-date calibre 551 in preference to calibre 561 and 564 chronometers. For example, throughout the sixties calendar Omega Constellations powered by calibres 561, 564 and 751 were produced in the hundreds of thousands (Calibre 561, for example, topping the one million mark) whereas the production of calibre 551 chronometers was a meagre 37,000 pieces in total.
The vast majority of calibre 551 movements appeared in cases 14381, 14766, 14900 and 167.005. The bulk of production was in the early 1960s, with 20,000 pieces being part of a larger unbroken production run of 100,000 chronometer grade movements in 1960. Simple maths tell us that overall numbers of the non-date calibre 551 in each model number are comparatively few when compared with calendar models of the period.
Also, in 1966, model 167.005 was replaced by the ultrathin model 167.021 powered by calibre 712. It was promoted quite heavily as a "dress" style chronometer and competed directly with models 167.015 and 167.025. The monocoques, while available, were not marketed to any great extent.
As the 60s matured, even fewer non-date models were produced and generally they needed to be ordered from national Omega agencies. That is why you will rarely encounter the 167.015 and 167.025 monocoques, most especially in 18k solid gold. Are they rare? Well, I would speculate that in 1966 when these models were offered only people who bucked the date and emerging day-date trend would have been interested in acquiring a non date model. Perhaps a few aesthetes with a post-modernist bent would also be attracted by the simple and comparatively minimalistic dial treatments, but production would have been quite low. So, the answer to the question of rarity, given such a small global inventory, is, yes, they probably are.