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:

Two formats of date code were used as shown by Sean Brown - basically chrome-bumper era on the left (1966) and rubber-bumper on the right (1974). However he assumes that on the right 'K' represents October as 'I' was not used for month 9 i.e. September as it could be confused with the digit '1' (even though it didn't cause a problem in the earlier format). But as digits were not used for the month in the later format, and the two-digit number clearly indicates which way round the code should be orientated, so 'K' could equally well represent December:

Bee's 12H2709 (casting number) Gold Seal head - 7E3 (1973) on a 17D2 (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 one has '70' at lower right indicating the year marker changed to a single digit in 1971.

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: