the wheels on the front of the one completely in the photo are missing the Diplock Pedrail parts, and since it's been forever since I've posted about what Diplock Pedrail is, http://justacarguy.blogspot.com/2011/08/pedrail-system-by-diplock-in-1904-pages.html
Shas (thanks!) identified it, it's a WW1 Italian standard 305mm howitzer, mounted on a De Stefano chassis and rail system which helped with the recoil and eliminated the need to dig a hole for recoil clearance.
Designed in 1908 and built between 1914 and 1917, a total of 44 were built by Armstrong-Pozzuoli and Vickers-Terni.
here are the common images found around the internet for years
The barrel is seventeen feet long and it stands, ground to trunnion about 12 feet. Weight was around 34 tons!
the model makers do such a damn good job, I'm posting these images to show in great detail what these looked like
Update Mar 2018, those are not Diplock Pedrail, they are ' Bonagente ' tracks and the cannon is an Italian Army Obice da 305 D.S.