This 2 register Sigdin chronograph from the 50's with push buttons. Not sure unusual is the right word, but scarce information about this brand. appears to have a landeron movement, though still narrowing down which one. 248 is my best assessment at this time, but just as easily "222, 223...you know...whatever it takes..." Does not, at the moment, function.
Elvstrom’s Yacht watch was an ingenious adaptation of the Regate watch originally produced by aquastar whereby it not only counted off the time with the minute discs and a central second chrono hand , which by the way reverted to a normal second hand , from the ten minute gun so as to arrive on the start line as gun gun went off but also to pre plot your course to steer by compass from mark to mark whether the wind is aft or forward of the beam. This is a titanium case which I understand was the last model before Paul Elvstrom jumped ship from Omega to Rolex and Omega recalled all .
The Leonidas Bund chronograph was regulated in 'Sidereal Time' and used for the navigational purpose of the German Artillery together with an Ulysse Nardin box chronometer to determine the 'True North':
"The mentioned Artillery were Pershing II midrange missiles with US owned tactical nuklear heads deployed in Federal German bases in the 70s."
"Sidereal time differs from solar time mainly because sidereal time is based on Earth's rotation relative to the fixed stars, while solar time is based on Earth's rotation relative to the Sun. A sidereal day is about 23 hours, 56 minutes, and 4 seconds long, which is approximately 4 minutes shorter than a solar day of 24 hours." => With this indication astronomical calculations are easier to perform.