I was going to suggest a No. 16 until re-reading your post.
I wonder if it's a No. 29? Do you have one for comparison?
Actually I was searching on Lexpev.nl but No.29 is missing there so I can't really compare... I also looked at countries who were using Lemon-type grenades and so far no results
Have a look at page 42 in this link Bombs and Hand Grenades 1919
They call it a British egg grenade and show a basic diagram, with a bit of luck it'll match yours, if not well, it's an interesting book but a pity about all the ads.
Had a look, it does look similar, but it doesn't look like there is adapter on top and the bottom also does not look similar..oh well, must keep searching