Minerva referred to their calibres by size, in ligne, and sequence number. Therefore all calibres sized around 13 ligne would be called “Calibre 13” followed by a number. The first 12 3/4 chronograph calibre from Minerva was number 20, so it became known as “Calibre 13-20”. It continued in production through the 1960's and was remanufactured using original equipment until 2000. At this time, an updated version was released and named “Calibre 13-21”. Number 22 followed shortly after.
Introduced in 1923 in Minerva-brand chronographs, Calibre 13-20 was designed as a mono pusher but was available after 1940 in bi-pusher configurations with 30- or 45-minute counters. It was one of the few chronograph calibres on the market on its introduction and was developed in collaboration with Dubois-Depraz. It was remarkably well-developed for the era and continued in production for decades.
Watch enthusiasts are able to recognize Minerva's Calibre 13 by the unusual positions of the pushers Rather than being located between the crown and lugs at 30°, they are moved toward the lugs at 37° from the crown. Calibre 13 is a column-wheel chronograph, 6.4 mm thick, with a Breguet balance spring and 17 jewels.
Even after the introduction of the updated 13-21 derivative, Minerva continued occasionally to produce (or re-manufacture) Calibre 13-20 movements. For example, Panerai used a re-finished Calibre 13-20 in their Panerai Ferrari 00024 chronograph, branding it “Panerai OP XXVIII”. Panerai also used this as their OP XXV in the 2011 PAM300 Mare Nostrum, though perhaps this was the later 13-22 instead.
On taking control of Minerva from the Frey family in 2000, the Minerva technical staff concluded that, although the old calibre design was still usable, it could not be produced with modern equipment. For this reason, the calibre was scanned and reconstructed in CAD for modern CNC machining.
The result was Calibre 13-21. Although apparently similar to the older Calibre 13-20, this new version is manufactured to tighter tolerances and includes modern updates. For this reason, there is very little parts exchange possible between the two calibres.
Calibre 13-22 is a vertical cluch column wheel chronograph. It measures 12¾ lignes, has 22 jewels, a Glucydur balance, and 18,000 a/H. It features a swan's neck regulator and has power reserve of 55 hours. It is decorated with Côtes de Genève on the bridges.