Some comments on the Mortier de 280 mm TR Schneider:
-they are only two typs of mortars: the first built for Russia and the french typ.In 1914, a french-russian agreement partaged the russian batch, the 8 first built mortars were allocated to France and after more were built, the Russia had his first mortars in 1915.The french and russian typs are near equal, they are some differences in breech and some minors details.
-the "small wheels" noted in a question: these wheels were removed after the mounting of the mortars, their use is for the manoeuvres to assemble in place.
-the wheels of the waggons are of three great typs: first in wood and steel, after all steel and at last all steel with caoutchouc tyres.
-the serial mortars were delivered from the end of 1914 to november 1918, in all 152 mortars 280 mm TR Schneider were built: 126 for France, 26 for Russia.The russian mortars were delivered in 1915-1917 and the delivery was halted with the russian revolution (they were 72 mortars foreseen for Russia).
I have writed a paper with many details and photographs in french review "GBM" edited by "Histoire et Collections" N°97 dated july 2011.