Your sight is worth in the range of $150 to $250 "depending on" who wants it, some sell for $90, others pay much much more. Looks to be in good condition.
Description from ht tp://www.rifleman.org.uk/PH_Service_sights.htm
THE MODEL 4 APERTURE SIGHT Folding Model For rifle No. 4 Mk. I
This compact and neatly designed aperture sight was ntroduced at the 1946 Bisley Meeting where it was welcomed because it was the one thing needed to make the new No. 4 Mk. I
rifle into a target rifle, suitable for use in N.R.A. (SR.b) competitions. As illustrated and originally sold there was no windage zeroing adjustment, but now all sights have a neat adjustable windgauge scale plate. It has the usual vernier elevation and windgauge movements. combined with distinct half-minute clicks; the six-hole eyepiece shown gives a choice of six apertures from .03° to
It is a part of the design that this and its military counterpart are sights pivoted and retained in the up or down position by the action of a spring-loaded plunger operating on the foot of the leaf. On recoil the sight is forced out of the vertical position but recovers due to the operation of the spring and plungers. Complete freedom of movement at the hinge is essential and this can be assured by careful fitting in the first place and a simple test may be imposed by pushing the sightt forward approximately 10' and allowing it to recover its vertical position, noting the complete absence of friction and the recovery of the sight to the correct upright position.
Since these sights must of necessity be fitted to rifles made under the stress of war con¬ditions in various factories, it may be necessary to ease the excess metal from the pivotal points on the rifle; another feature to note carefully is the fit of the plunger in its guide hole. With proper attention paid to these details this Model PH.4 sight need never be blamed for its short¬comings as an aperture sight of excellent merit and serviceability.
