My interest in “Type XX” timepieces is exclusively the 3-register flyback chronographs manufactured by Mathey-Tissot for Breguet, Girard-Perregaux, and also under the Mathey-Tissot brand.
I have never seen published material from the Girard-Perregaux or Breguet indicating that the pieces were manufactured by Mathey-Tissot, though this is accepted within the community. Similarities are so extreme between the Mathey-Tissot and both the Girard-Perregaux and Breguet that I believe both to be manufactured by Mathey-Tissot until shown otherwise. Girard-Perregaux does not have any information in their records regarding a relationship with Mathey-Tissot  and nor does Breguet.
You may read that the Girard-Perregaux is the most rare of these 3-register Type XX, though without primary source material from Mathey-Tissot, this is debatable. To date I have found slightly fewer pieces branded Girard-Perregaux than Mathey-Tissot, though not by an amount that I consider relevant to support the regular claims about the rarity of the Girard-Perregaux (though I fully appreciate that sellers do this in order to push prices higher). Breguets are by far the most common of the three, though command much higher price points (their military pieces for the Centre d’Essais en Vol (CEV) driving this). With relatively few pieces seen for Mathey-Tissot and Girard-Perregaux, we see prices climbing correspondingly higher.
What makes a Type XX?
These 3-register pieces build upon the initial specifications of the Type XX, which were first manufactured for the French Army in 1954, and are believed to include the following characteristics:
– black dial
– Arabic numerals
– luminous hands and indexes
– seconds chapter ring
– screwback case of about 38mm and less than 14mm thick
– mechanical self-winding chronograph movement with a power reserve of 35 hours
– accuracy to within eight seconds per day
– flyback mechanism
– two registers: at 3 (to measure 30 minutes) and 9 o’clock
– 12-hour bi-directional rotating bezel
– evidence of ability to successfully start, stop and reset the chronograph 300 times.
The 3-register evolution of the above specifications additionally included an hour register, and either a 15 or 30 minute register.
In this review, I am researching those pieces that specifically have the same case design, not a compressor-style case design, where the outside edge of the lugs are more vertical in design.
Are these watches really Type XX?
For ease of discussion, I am going to refer to the category here as 3-register Type XXs, though the Type XX specification was strictly a 2-register military chronograph , not for civilian or 3-register pieces we see later.
The model “Type XX” (and “Type 20” from Breguet) was (and still is) used as a model name for later pieces that do not follow the initial specifications as manufactured starting in 1954. So consider “Type XX” on this site to colloquially refer to the category of timepieces that came in the late 1950s and 1960s, both military and civilian, with 3-registers and steel (as opposed to bakelite) bezels.
Based on dating available from Breguet, it appears this group was manufactured starting in 1959 . No information is available from either Mathey-Tissot or Girard-Perregaux for these pieces, though the hope is that there will be information published by Breguet around the Type 20 (military) and Type XX (civilian) in 2019 or 2020 based on research by Emmanuel Breguet.
There are two websites that I recommend reading to learn more about the history of these pieces:
1. Don Indiano’s website on the history of the Type 20, and
2. The Springbar: Breguet Type 20 Generation 1 Military Issue and Breguet Type 20 Civilian, Generation 1, 2 and 3
Outside of this research
This work does not touch upon the 2-register Type XX as produced by Breguet, Vixa, Auricoste, Dodane, Airain (manufactured by Dodane), the non-existent Boullier, or the 3-register Dodane.
 Initial date of manufacture based upon the Breguet Type 20
 I have never seen the source document from the French military that specifies the requirements of a “Type XX.” Please let me know if you have a copy or know where I can locate one.
 Thanks to Wally Schweizer, Museum Curator at Girard-Perregaux, for information on company and timepiece history.