Rough guess mid to late 1950s.
The stamped letters/numbers at the rear would give us the best clue, but I can’t read the prefix letters.
For lack of any better information we are calling those production sequence numbers. They started putting those on a plinth as you have after '52 and before '57. Before that they were just stamped into the head at the rear, and even before that they were stamped in the middle of the valley.
Also I see you have the later type hex plugs where earlier heads had socket head plugs like my '52 head.
The cast C and stamped 6733/1 is a bit suspicious as this is the part number and was always fully cast numbers, not stamped.
But perhaps it was stamped because the sand was too dry that day and the numbers didn’t form right in the mold and it needed a part number for spare parts inventory keeping. That number was used on 2.4 L Mark 1 saloons.
The letter A is interesting. We often refer to this head as the A head because it came before the B head and the C head, but I haven’t seen one with a letter A before.
The A heads were always unpainted raw cast aluminum, not painted silver.
Hi Rob I just had another look at the stampings at the rear … there is no prefix letter… there is what appears to be a witness mark of the head of a slotted screw where the prefix number should be. so all there is is the number 114
I did a bit of checking in parts books, and the only models that list the C6733/1 head are late Mark VII and 2.4 Mark 1. It is not listed for any later models.
Late XK120, mid range Mark VII, early 2.4 Mark 1, and XK140 list the C6733 head without the /1 suffix.
The suffix /1 probably indicates some small improvement that was made around 1956-57 or so.
But since yours has no serial number, it was a replacement service head suitable for all those models.
The mark like a slotted screw head must be some poorly stamped or damaged letter or letters. There would not normally be a screw head there in the sand mold.
Can we get a photo of the inlet ports on the side of the head? I know a guy who was able to source a replacement head for his E-Type. That head had been in a dealership’s parts inventory for many years. It too has the raised “A” casting in the spark plug valley. I’m wondering if your head might be a straight port head. My guess is these heads with the “A” casting in the spark plug valley were produced after-the-fact to be sold as replacements.
Hi Mike interesting… I`ll get some images later today
Remarkable what is still around… Mike has asked me to post some images of the inlet ports which I will do later today… Thank you Please let me know what you think once you see the next batch of images
That is definitely the inlet face of an A-series or XK120 head. You can tell by the two small square water ports. Those square ports were enlarged on the later B-series heads.
What is different on your head are the hex plugs in the spark plug valley. Those are the type used on the B-series heads and later yet straight port heads.
Also the raised cast “A” in the spark plug valley is unique. As I said yesterday, my guess is this head was cast after-the-fact as a replacement series head.
I want to correct my earlier statement. On second thought, those square ports were not enlarged on the later B-series heads.
Thank you Mike so likely a factory service replacement from '56/7 for the 120, & 140. Can’t be many left NOS. Just a pondering… if this was a factory replacement on an otherwise matching numbers car…does this maintain its status ?
In principle could this also be ‘Ctype’ ?
is it possible to get a nice crisp clear pic of the number at the back of the valley and the part number stamped on the underside C6733-1
thanks terry
Hi Terry… I’ll do what I can
The last model to use C6733/1 was the 2.4 Mark 1 which ended production in 1959. The suffix /1 after C6733 indicates that it was made some time after the public introduction of the 2.4 Mark 1 in Sept 1955 as the first 152 Mark 1s used C6733. So that puts it in the time frame of late 55 to 59. Or possibly a leftover from after the 2.4 Mark 1 ended production.
So it could not be originally from a C-Type, and not likely to be selected for a C-Type replacement as by that time the C head was in production.
The prefix letters at the back plinth are most likely a letter B and a second letter A-Z but I believe I and O were not used.
The hex head core plugs replaced the hex socket plugs in the 1960s but they apparently did not change the part number C.2157 nor even add a /1, just the note in later parts catalogues for Mark X and 420 that it was Core Plug (Headed).
But this head appears to have been sand blasted so we cannot assume the core plugs are original to it.
The cat agrees. 
thanks for that
terry
something I only just noticed… look at the dash… then look at the 1… they are the same stamp .
There’s another NOS C6733/1 head currently on eBay. The 6733-1 numbers on the eBay head are also stamped - not raised cast as would be expected. Additionally, the eBay head has the raised cast “A” in the spark plug valley.
$8500???
Are they joking… or are these now that rare?