Hi,
I'm not so certain it's for a depth charge thrower. Please consider the attached images of a DCT case I have. It is basically a 40x158R case, marked "2Pr No.1, 1942", but the case has been cut down (to 125mm) and has been re-stamped in June 1944 for the 9.5" Thornycroft depth charge thrower (marked as such at the 12 o'clock position). The primer is missing.
I hope the markings are clear as I had to drastically reduce the image size AND quality in order to fit the 97.7KB JPEG file size limit (yes, sorry guys, this is a bit of a BOCN pet peeve of mine; hint, hint.

). The 9.5" part is difficult to read, as the "9." is very faint.
The other picture is of interest too, and clearly shows a phenomenon for which I'd like to see an explanation myself: the shortening and the bulge near the upper part of the case. I've tried to find out if the case was cut down in order to be used for the depth charge thrower. The only thing I managed to find out was that the case should be "somewhat adapted to fit a spigot mechanism". I hope it has the proper length.
Anyone who knows for sure?
Cheers,
Olafo
PS: For what it's worth: when trying to research this case I think I came across two different DCT mechanisms using 2Pr cases. I'm not certain what the cases for the other type would look like, and if they'd be similarly marked...