Perhaps this should go in the FAQ section, but I'll leave the decision up to the moderator...

As a collector primarily of the Seiko & Citizen 60s-70s "1st" & "2nd" generation automatic chronos, I got to wondering about the relative rarity of each calibre and of the various case & dial styles. Since Seiko won't give out production data (not sure of Citizen), I have to rely on what I see offered for sale various places or up for display on various Japanese watch collection websites. I then infer the relative rarity of each model based upon how often I've seen it over the years I've looked. The logical 'flaw' in this method could be that certain models are around in peoples hands BUT just not put up for sale or 'show' on the net.

From my own informal research in this way, and assuming that frequency of occurence on the net is a good approximation for relative rarity, I come to the following conclusions:

SEIKO:
Calibres available: 6139A (21 jewel), 6139B (17j), 6138A (21j), 6138B (23j), 7015A, 7016, 7017A, and 7018A

1. The rarest models in my experience are ANY one with a calibre 7018, 7015, 23 jewel 6138B, or 21 jewel 6139A. I include in this category anything in a "Time Sonar" style case (calibres 7015 & 7018 with expo back & possibly a see-thru dial) AND anything with both a sliderule under the glass & external tach bezel (models 7015-7010 & 7017-6050). Also, in this category are any 6138 or 6139 with ALL of the following on the dial "Seiko 5 SPORTS Speedtimer" and a 70Meter Waterproof indicator. Another one in this category is a 6138-0020 (that has the case style known as 'helmet', 'flightmaster' or 'volcano'). In each case I've only seen 1 or 2 of each on the net (either in a collection or for sale) over the last 2 years.

2. Next rarest are 6138-7000 Sliderule, any 7016, and any 7017 (other than the 7017-6050). Also in this category, ANY 6138 or 6139 with "Seiko 5 SPORTS Speedtimer", but NO waterproof indicator on dial. Examples are 6139-8050 or 6139-8002. Also in this category is 6139-7080 (with octagonal case). In these cases I've seen only a maximum of a dozen of each pictured in 2 years. To put this category in perspective, the items listed here are seen about as often as the 1960s era manual "Crown Chrono" using Calibres 5717A, 5718A, or 5719A. 

3. Next rarest are 6138 & 6139 models with only "Seiko Speedtimer" on dial and also the 6138-004x Bullhead with Black dial & Blue subregisters. Generally a max of about 2 dozen seen in 2 years.

4. More commonly are 6138 Brown/Gold dial Bullheads with just "Seiko Chronograph Automatic" on dial and also 6138 models like the 6138-3000, 6138-8020, and the BigSmall Eye (-0011 & -0017). I'd also fit into this category any chrono that has the waterproof indicator on the dial in addition to the standard "Seiko Chronograph Automatic" marking.

5. Beyond that I regularly see a good number of 6139s like the 6139-8020, 6139-7100 ('volcano' or 'helmet' case), & various 6139-60xx styles (where 'x' represents any digit). The 6139-60xx models with the red & blue external tach bezel actually appear quite common (although generally are quite in demand). In this category, they generally just say "Seiko Chronograph Automatic" on the dial.


CITIZEN

Calibres available to my knowlege are the 5700 (actually I think this is manual), the 8100A (equivalent to a Seiko 7018A), and the 8110A (like a Seiko 6138B. The 8100A & 8110A both have 23 jewels & Flyback complication. I'm not sure if 5700 has Flyback, but it has 21 jewels.

1. Rarest appear to be 5700 RecordMaster, particularly the "Rally Custom" having a black external tach bezel AND a sliderule under the glass. Also, any 8100A. Outside of Japanese collection websites, I've never seen any 8100 or 5700 "Rally Custom" offered for sale in over a year on the web. In fact I only once saw a 5700 (needing mechanical work) offered during that time.

2. Next rarest are various non-bullhead style 8110A chronos. There are some that have an external enamel tach bezel & have the general appearance of a boys sized Omega Speedmaster. Others have octagonal cases & hexagonal subregisters, others are more square cased with fancy shaped chrome bezels. I generally see a handful of each for sale in a year.

3. Believe it or not, the 8110A bullhead styles actually seem to show up most often for sale, though the hexagonal bullhead case style is rarer than the 'rounded' bullhead. Another more common one has a somehat 'volcano' shaped case and an inner rotating bezel (marked in 60 increments) with a separate crown @ 10 o'clock to turn it.


FINALLY: since these conclusions are based just on a few years of personal observation of "what shows up" on the net, it is subject to error. But, in any case, I think it gives a good idea of how hard it would be for a collector to actually get his hands on a particular model of vintage chrono.

NOW - have YOU come to the same conclusions I have, or have you gotten a different picture of the situation from your hunt for these items for you collection?