While this is almost certainly a homemade "Bulldog" type cut down no.4, I have seen images of a relic No.4 barreled action dug up on a WW2 European battle field that had been shortened in the same manner and a Grenade launching tube afixed to the muzzle.
Speculation at the time was that rifle had been a one off modification for a special purpose, perhaps as a compact anti-tank grenade launcher, or for use in launching heavy smoke grenades to mark a LZ.
British Paratroopers had been engaged in battle on that spot, but little other information was available.
If the cut end of the fore end and handguard show raw wood its most likely a very recent modification. If the cut ends show significant aging then theres a slim possibility that the alteration may have been done while the rifle was in service.
Cutdown rifles are a fairly common concealable self defense weapon in a number of third world countries. Among confiscated weapons in Pakistan and Afghanistan were many cut down 11mm Mausers and other shortened rifles.
I've seen images of many home Bubba'ed cutdown No.4 rifles over the years, and at one time a similar but longer barreled "Bulldog" was offered for sale by importers.
There had been several types of prototype Bulldog conversions of the No.4 while the No.5 carbine was in development stages, but so far as I can tell these are all in museum collections.
Similar very short Bulldog conversions of the M1917 rifle are among the more oddball exhibits at the Springfield Armory museum. There used to be a photo collection of these exhibits online, but that site went down many moons ago.