Projects by the Swiss, Czech-born engineer Vladimír Skála for the light engine M-137 and its derivatives, in all pictured cases three-piston "half-M-137". Total 12 projects, which he used as ilustrations in his self-published calendar. None of those projects ever left his work table (except for the calendar, obviously), all were just for his fun.
KF-2 Kurýr (Courier) - upper left
KRT Kondor (Condor) - lower left
RS-4 Rorýs (Swift) - upper right
FR-2 Frajír (Lad, in Moravian dialect) - lower right