No4 Mk1 Maltby Rear Sight Question

[Monkeytwo]

I have acquired a No 4 Mk1 (Maltby) as a restoration project. The original rear sight (Mk3 stamped) was damaged and I purchased a milled rear sight (B stamped) as a replacement. I found out that the B stamped milled sight will not fit on the Maltby. pinning it to the rifle is no issue, but when the sight is flipped down, the top edges of the sight are slightly larger that the recess and the rear sight will not fold down all the way.
I have seen pics of No4 Mk1 with milled sights, stamped sights, flip sights, but wondering why this milled sight will not work.
Milled sight was purchased by Numrich (Rear Sight, Milled, Blue Steel (800m; Micrometer Click Adjustment For Elevation)

I am looking to replace the milled with the stamped rear sight but any ideas why the milled would not work ?

Thanks in advance