12Pr 12Cwt Primer Adaptor

What is the correct mark for this brass primer (vent tube) adaptor?
It is 60.0mm long (2 3/8") and came from a 12Pr 12Cwt WW1 case.
It looks like a Mk11** by EOC that is cancelled and stamped RL 1 which seems strange.
Mk1 adaptors are not threaded on the end.
I use D.Ibbetsons book “British Artillery Ammunition” vol 3 page 92 as a reference.


Is that EOC looks more like BOC to me. If that makes any difference.

It is Elswick Ordnance Company. Common on many artillery headstamps.
Mk11** adaptors were not listed in the book, only Mk11.