Unknown threaded hole in head below diesel return outlet

[redbear]

Hello Fordson Enthusiasts,

As a brand new newbie, this is my first post. I apologise in advance if this question has been asked previously, but I could not find any specific reference to it when I searched.

My father has a fordson new major diesel, which has sat for 30 years after a rebuild. We got it running yesterday, but the cylinder head appears slightly different to all other pictures I have seen. Below the diesel fuel return port at the rear right side of the cylinder head (drivers seat perspective) , there is a large threaded port that does not appear in the parts manuals. I 'think' it was a mounting point for the rear compression mechanism, but as the rocker has a front mounted compression lever and a new rocker cover , this rear port appears redundant.. My questions are, can anyone tell me what this rear port was for, and is it ok to simply plug it up with a plug ?

If pictures would assist, I will gladly post some...

Many thanks for your assistance and time,

[Brian]

Mk 1 Decompressor lever used that hole on Majors from 1951 to 1955 approx.