Some detail - the 3, 4 and 5 shells are identical in form, but the mk3 has the chinstrap rivets fixed noticeably higher up the side of the shell, and of course usually (not invariably, but almost always) has a simple nut and bolt fixing for the liner.

The 4 and 5 (and some retrofitted 3s which have a LTD spike welded into the crown) have the lift-the-dot liner. The liners for the 3, 4, and 5 are all different, although that of the 4 resembles, except for the LTD fitting in the crown, that of the 3. Which is of course the same liner as used in the mk2 (during WW2 mk3 were usually issued without liners and troops expected to use the liner from their existing mk2 helmets). The liner for the mk5, which of course will fit any LTD fitted helmet, is very different to the previous version. Excellent breakdown of this liner on, of course, Joseba's webpage - Ingls MK5