OK - not so sure about the ribbed-cam cover aspect, but indeed may have something to do with the USA 1968MY package (Emissions and Safety Legislation, and how British Leyland handled that).
First up - see below a copy of Service Bulletin B.71, which as suggested really only talks about the Block stamping of Engine Number……….
But my other source of factory information is the Series 2 E-type, owners Handbook - the next scan shows page vi as in issues E.154/1 to E.154/3 of the handbook………… note, talks about both block and head engine number stamping.
Then the revised page vi for handbooks issues E.154/4 and E.154/5 now reflecting the advice as per Service Bulletin B.71, but simply as (Early Cars) and (Later Cars) without the B.71 exact Engine Numbers………
Now this is the situation I am familiar with after many yeras of looking at predominantly RHD Australian Delivered Series 2 E-types (and Series 1-1/2 E-types) that did not have of course the USA only legislated Emissions Requirements, and all the associated regulatory bureaucracy in obtaining legal entry to USA market as USA 1968 MY cars (the S1.5 cars) and then as USA 1969MY cars (the S.2 cars). So maybe the deletion of the Head Number stamping for USA cars only, was associated with the head modifications required to comply with the 1968MY emissions (and regulatory) requirements, and if so, then look for physical examples from late 1967 date-of-manufacture cars, with exact introduction point of 1968MY compliant cars being of a known Engine Number. (I do not refer to 1968MY cars as being the introduction of Series 1.5 as in Australia and other non-USA markets Series 1.5 refers to the first removal of the glass headlamp covers, which is some months earlier in 1967)
With an International Forum such as this, we need to be careful not to use parochial terminology, which starts to matter with 1968MY USA market cars and later.