Navyman. I've never heard of variations between filler plugs for the 5 and 23. Someone is getting confused with the 36 which does have a larger filler screw.
Main differences are the lever, which was normally the type with a ridge down the middle but I have seen 23's with flat levers too. Base plugs of course and the slotted striker as Andy points out. Skennerton talks of variations to the body casts but over time most people now put those down to different casts made by different makers, trying to follow the original drawings but introducing slight variations, mainly at the shoulders. Cast marks are no indication of grenade type. If so I have Mills No 5 bodies with 6, 8, 9 and various letters cast in them. They are just mould marks.
Hope this helps - John