Thanks Tony.
I did check your Cartridge Casing list, and indeed couldn't find it either... .
The inner opening on mine is 31 cm. And 34 cm at the base.
I had some help on a French forum where it was id as a 305mm shell.
But it rather seems to be a complex matter.
According to them they can come in different high. Depending on the range of fire.
And that for the same purpose extra charges could have been added to the shell in order to send the obus on a wider target...
It is very well possible that my shell had been cut down, to serve as a umbrella stand, but then again it is strange that on the inside something had been provided to prevent that stick's or umbrella's would sink to deep into the stand...