Cylinder Head Identification

A friend was looking at my cylinder head and he noticed that it doesn't have air rail holes drilled for the air pump. Hmmm. Very puzzling. On the suggestion of folks at MGExp I pulled the valve cover gasket and made a note of the number. The head is a number 12H4736. Pat# 565394. This is actually a British Leyland produced replacement head for the B. I think they came without the air rail holes drilled so that they could be sold in both the US and UK market.

If you want to ID you cylinder head, here are a couple of useful sites...
http://www.flowspeed.com/cylinder-ident.htm
and

http://www.mgbmga.com/tech/mgb25.htm

Ray, owner of the famous supercharged B writes on his site...

The MGB engine used four basic cylinder head designs. The fourth version of the cylinder head was a slightly modified version of the 12H4736 cylinder head first introduced on the Austin Marina and used in the UK/European market RB MGB (74½-80) as Original Equipment.
This cylinder head was also used as a replacement cylinder for the CAM1106 on the US RB MGB (74½-80). They are essentially the same as the CAM1106 with slight modifications. It was referred to as the "lead-free" cylinder head and was induction hardened to withstand higher combustion temperatures of lead-free fuel.

The process involved 1% tin being added to the molten iron before casting, additional depth was used which was machined off before the induction hardening process was applied to the valve seat area, all valve seats. This was a complex process and applied only to the US market cylinder heads. It used 1.5625" inlet valves and 1.343" exhaust with a more efficient "Open" Kidney-Shaped combustion chambers featuring a larger squish area as well as reduced promontory between the valves, a chamber height of .375" with a volume of 39cc, (UK 43cc). The rear rocker shaft pedestal had an offset oil feed in order to accommodate the redesigned cooling passages that assisted in preventing overheating of the rear cylinder. These larger coolant ports also provided a greater service area to assist in dealing with higher combustion temperatures that resulted in efforts to reduce emissions. It has air injection ports and a water choke outlet at the rear of the cylinder head. The main difference compared to the CAM-1106 was a slightly more improved intake port design and the outer shaping of the head was revised giving a much smoother appearance. The Patent Number 565394 located by the thermostat housing was removed and marked inside the cylinder head.
We felt this cylinder had that little extra and when ported would provide us with low and middle range torque & power for increased street performance that we wanted.

Most likely the original head cracked and was replaced with this one. All fascinating stuff...