A Brief Explanation of Later Lee Enfield Foresight Blades.
A Brief Explanation of SMLE No1 MkIII & III* and Rifles No4 and No5 Foresight Blades. With Additional Notes on P14 & M17 Foresights
No4 foresight blades were made in 3 Marks (‘Mk’), although just to confuse things there was Mk1 and Mk1*, followed by Mk2 & Mk3 - so in point of fact there are 4 variations. Mk1's were solid base (dovetail) and Mk1* were split base. Both of these had the back of the blade come right to the back of the block base. Mk2 were solid based and Mk3 split based. On these the blade was undercut at the back (sloped forward from the tip) to give better definition of the tip.
Just to throw things a little there was also a specific set of blades for the No5 Jungle Carbine. They look like No4 Mk1* (split base) blades, although they are shorter front to rear and have their own part number sequence starting 'BJ' rather than 'BB' or 'CR' and these are designated 'Blade, Foresight Mk1'. Depending on the size of the hole bored through the foresight protector ears on the No5 foresight block, some No5 Jungle Carbines will accept a No4 foresight blade – some will not.
All of these blades are made in 0.015” increments, sized from -0.030" to +0.075" although Mk2 and Mk3, No4 blades, also have a 0.090" blade which was added to the series sometime after WW2.
SMLE (Mk3 & 3*) foresight blades are all solid base (dovetail). In form they are similar to No4 Mk1 blades, although generally the platform that the blade sits on is wider and consequently the dovetail part looks flatter and smaller although it is not. There are 8 sizes of SMLE blades listed, running from -0.060" to +0.045". Some references also list 2 further sizes +0.060” and +0.075” although so far I have not found a reference to British Army Part Numbers for these.
The '0' sized blade is a zero on both SMLE & No4 & No5 foresight blades because the sizes are taken from an imaginary line 1" above the centre of the bore, therefore on '0' blades the tip of the foresight is 1" above the centre of the bore, the +0.015" is 15 thou above the line, the - 0.030" is 30 thou below the line etc.
So, SMLE blades should be physically interchangeable other than the size ranges are different.
Just to confuse things slightly, P14 foresight blades are the same form as (and interchangeable with) SMLE blades - BUT - (big but) they are marked with different sizes because the dovetail on a P14 foresight is a different height above the centre of the bore, 0.060" lower to be exact. P14 foresight blades run from -0.015” to +0.135" and are marked as such. In other words a P14 blade marked '0' is the same size as an SMLE blade marked -0.060". P14 sight blades are usually marked ‘P14’ in addition to the size.
If you are not quite confused enough already; The US .30-06 M17 Rifle has 11 sizes of foresight blade which will interchange with P14 (and therefore SMLE) blades. These are also in 0.015” increments but based on an imaginary line placed 0.950” above the bore centre line. These range, and are marked, between -0.015” and +0.135”. Because of the differing ‘zero’ height and the different trajectory of the .30-06 round, making an adjustment to either a P14 or an SMLE using these blades is largely a matter of ‘suck it and see’!
I hope this helps? At this point I would take yourself off to a darkened room for a while and have a lay down. That is what I have to do every time this question comes up!