This page gives the chapter its name, 'Corridors of uncertainty'. It is explained here.
Composition, 1927 (both)
B184 and B186, both entitled Composition were amongst nineteen Mondrian paintings shown in a 1929 chair exhibition, photographed by Grete Leistikow (fig. 1).
The photograph includes the two lost paintings and the extant B185. Fig. 2 shows all three in black-and-white, and B185 in colour
B185 is 40x50cm and that allows estimates of 38x56cm for B184 and 50x52cm for B186.
The large coloured area top left in B185 is blue, suggesting that the large coloured areas in the lost works are not blue, but probably red or (unlikely) black. The only red area, bottom right in B185, is too small to read across the images.
The reconstructions are shown in Figs. 3 and 4.