Three combustion chambers from Lyndsay Porter's 'Improve and Modify MGB': On the left what he describes as a 'very early' head with a pronounced protrusion between the valves (A) which he says causes adversely affects gas flow, as well as a noticeable 'curl' (B) around the lower right of the plug hole. Centre a significantly reduced protrusion as well as slightly less of a curl, on pre-1970 heads. And on the right a later head with the protrusion reduced still further, with the curl reduced still further:

Two from Sean Brown's Flowspeed.com (re-orientated to match the others): On the left a 12H906 head he says were the earliest used on the MGB from 64-65, however this is more like Porter's middle picture. On the right a 12H2923 head Sean says was used from the start of the 1972 model year to the end of chrome-bumper production:

Bee's head. This is from a Gold Seal 48G733 engine equivalent to the 18V58n series used from the 1972 model year, 12H2709 with a 7E3 (1973) date code, used for the remainder of chrome bumper production. The protrusion looks to be less than Porter's middle picture but more than the right, with the curl about the same as on the right:

The Peter Burgess supercharger head. Protrusion about the same as Porter's third picture, but cut back significantly above the exhaust valve, as well as polished all over:

The two oil-feed arrangements (arrowed) from Sean Brown's pages: Original 'in-line' arrangement on the left, offset on the right with the CAM-1106 small inlet-valve head in 1974 on rubber-bumper cars:

The underside of an offset rocker 12H4737 replacing 12H389 on 18V797 and later engines - not all suppliers info is correct. With either mismatch (as well as being obvious from a cursory glance) rather than blocking the oil feed hole it's more likely that oil would escape from or near the base of that rocker:

Examples of head date codes. Similar to blocks, the year indicator changing from two digits to a single digit about the same time the engine codes changed from 18Gx to 18V. The 66 (probably) and 67 (definitely) use digits for the months whereas the (7)4 uses letters as per the BL date code document:

Bee's 12H2709 (casting number) Gold Seal head - 7E3 (May 1973) on a 17D2 (April 1972) block:

This is an example of a 12H4736 casting dated 1977. It is from an old eBay ad for a head purporting to be from a 1974 engine, which just goes to show you can't assume what is in your car is original or even correct:

A gallery of heads gathered from around the internet in approximate date order. Note the second and third are '70' and '71' whereas the next is '2' for 72, and the month changing from digits to letters i.e. the change from two digits for the year and letters for the month didn't happen from 1970 as the BL spec indicated:

No date code on these three, the final one showing it is a late head with the offset oil feed to the rear pedestal:

The following table has been derived from the Parts Catalogue and Sean Brown's Flowspeed.com page, but note he majors on engines for North America. There are some inconsistencies with dates and descriptions as indicated:

Air injection ports (plugged) on what is probably a 1971 18GK engine. Note that '71' is shown so either the change to the head castings was done later or this one was right on the cusp of the change: (Stephen Howden)

Information on plugging air-injection ports from Sean Brown.