No 4 Mk1 Trials Rifle Rear Sight Plunger & Spring

[Surpmil]

Here's a scan of a print from the film camera days that shows the difference. The rifle on the left is the trials Model C, of which 56(?) were made if I remember correctly. Notice that the cut made to accommodate the larger and later standard plunger is "in the white". The slightly odd residual "lip" at the top of the bolt way on the right side is thus explained!

And here is a crop of a Model C trials No.4 being shot at a range in the UK. The dowels inserted near the forend crossplate were a civilian modification. Notice the sling swivel on the guard. The forend cross plate is also differently arranged than the standard No.4.

[breakeyp]

Ball size: use a set of drill bits to find one that fits the hole or fitted to the notch in the bottom of the sight. That will give you a diameter.

[Mk VII]

There ya go :-

[waco16]

Thanks MkV11, but that's the std production No4 plunger
The trials sights had circular cut-outs rather than flats for the plunger:

[Mk VII]

These blueprints (and for once they really are blueprints) are undated but a minute sheet in the file, of May 1932, lists a number of design modifications to be discussed including " .... new type spring & plunger for screw, backsight - plunger in place of ball. Detail 28, 47, 70 [appears to be the dwg numbers]. CISA proposal. Local agreement clear. " So they must have changed it quite early on - or is that just the clicking adjustment?

[Surpmil]

Interesting as the Model C I had was dated 1935. Slow process I suppose.