A decent quality switchfuse in nice condition there. Would you get away with displaying it on the mantelpiece?
When that was put in, the main domestic applications for a 30A switchfuse would be cookers and submains, as there were no 30A socket-outlet circuits (15A outlets were always on 15A radials). When used for cookers and similar, prolonged heavy load often caused the wax plugs in the fuse carrier screw holes to discolour and distort or even start oozing out, so I am going to guess it was feeding a submain with modest load.
As a rule of thumb, DP fusing was standard before the war and SP from the beginning of the 1950s, because the change in regs allowed the assumption that the neutral would be solidly earthed and therefore did not require fusing. There were probably a lot of DP units still being fitted in the interim though, and odd supplies that still required DP fusing for long after. These days one is used to viewing anything with a fuse in the neutral as a hazard and a defect but it was standard and necessary up to that point because of the risk of a neutral fault to earth causing an overcurrent, if there was a significant potential between them. Many DP units remained in service with fuse wire in the neutral, although in industrial installations under the care of an electrical dept some did have solid links fitted.
I used to work in some locations with mainly prewar wiring right up to the millennium, including mainly DP switch and fusegear. I routinely put heavy wire in the neutral carriers, heavy enough to discriminate with the line fuse when clearing a fault.
A final interesting thing there is what looks like a screw-grip conduit coupler in the top. This was the forerunner to Conlok and was widely used in small sizes - 5/8" and 3/4" for domestic work, but less for the larger conduits (is this one 1" - it looks like it in the pic).
Did you see much of this type of conduit in the installation?