Unfortunately, it is one of a series of captions of about the same quality in the CWM, and they don't seem to want to know, either.
Wrote to the Director several years ago about a few of their display captions, suggesting further investigation with a view to correction if they agreed with my comments and evidence provided.
No reply. Not a squeek. As far as I know, no caption change either. Every museum makes caption errors: the real test is in correcting them.
Check out their 18 pdr shrapnel shell caption. It's a real beauty!
As for the No.5 Grenade, which is similar to the No.36 or 36M, one of the tell-tale difference in the body casting is the flair in the bottom below the segmentation, which is correct for the No.36 or 36M, but not for the No.5, which is segmented all the way to the bottom and does not have the flair. The top casting surrounding the striker is also different on the No.5 compared to the No.36: the No.36 has a smaller casting that does not extend past the striker pin.
So I think this is a No.36 or 36M, not a No.5. And as Ed has said, the No.5 had a 5 second fuse ('detonation within 5 seconds of the release of the lever' the spec reads), not a 4 second fuse on pulling the pin!
Mike