Mk I sight swinging on the leaf

[bow]

if you have a UK made Mk3 or 4 backsight fitted, then these sights require a SPACER, pin axis backsight to be fitted too. Without this, the sight can and will move from left to right and vice verca.

http://www.euroarms.net/EFD/manuali/...RE-IT-1948.pdf
TAB. B shows nomenclature of backsight, what letter identifies the spacer we are speacking about?

To give an idea the clearence i have here i nedd a thickness of two paper leafs used for computer press.