The 'cracked skull' badge was in deed a great mystery within No4 Commando and many veterans themselves were unaware of its existence until quite recently. The badge was introduced for Officers by Lt Col CPD (Percy) Legard, of 5th Inniskilling Dragoon Guards, the Original Commanding Officer of No4 Commando. I believe his tenure as CO was relatively short lived as when No4 and No7 Cdos were drawn together in Nov 1940 (as 3 Special Service Bn) Lt Col D.S. Lister MC of the Buffs was in charge of the joint 'Battalion'. By March 1941, when No4 and No7 Cdo resumed their distinct 'Commando' identities Lister was in Command of No4 Cdo with Lt Col JB Colvin as CO of No7.
Alas it appears this badge has now been reproduced. However the correct pattern is found in our CVA insignia album as the correct 'cracked' skull. The following link relates.
There is a much more round, squat, bulbous example in the same picture gallery - which lacks the crack - there is little doubt (at least in my mind) that this is 'spurious'.
The correct pattern was illustrated on the lid of a Cigarette Case retained by Percy Legard's daughters some time ago in an article of the Military Historical Society 'Bulletin' ... emphasis was placed on the 'Crack' found in the skull, and apparent on your excellent wall shield.
Collectors beware though - there are badges being manufactured in America (I think) - of relatively poor modern construction but with a crack in the skull.
It appears that the correct original badges were manufactured by a local jeweller at the request of Percy Legard - very few have been traced.
Mike
I would be pleased to learn more about this badge as it is of great interest