[Lynn Eades]

From the info gathered, I would conclude that at some point toward the later stages of production, the tooling for the MkII's was altered to allow for the more simple construction by use of a flanged pipe fitting. This was obviously continued through out the rest of the Canadian and British production. (MkII, MkII*, MkIII etc) The two change dates may not have coincided.
I remind people that many MKI's and MkI*'s were converted to MkII's etc. (See Nigels books)
There will be a lot of MkII's that have the oval hole.
My Dagenham built Ford Armoured OP (T84991)was the 370th, of a batch of 3,443 (T84621 to T88063) Production ran from the 17th september1941 until 16th march 42. I suspect the whole run would have been tooled the same way, and so the round drain arrangement was probably in place at 17th september 1941. The first MkII U.C's were in a batch numbered around T63xxx. This accounts for a likely time frame in the U.K.
I have no time frame for Canadian production.