The Overprint Fonts and Layouts
The only major variety that has surfaced on this type of font is known as the "fishhook G". It is very rare, and so far it is only known to occur on the upper right pane in position 5 on the 50c light green. It is distinguished from the normal flying G by the fact that there is almost no serif at the top of the G, ans the crossbar of the G is almost entirely truncated where it meets the vertical bar.
Although I have not seen this variety on the 14 point Casson font, it could well exist and you should look carefully at all your stamps.
Over the past few years there have been unscrupulous sellers on E-bay that have been altering cheaper unoverprinted sheet stamps by adding in O.H.M.S. and G overprints using a laser printer. At the same time in reaction to this there have been a very zealous group of e-bay members who insist that they can determine the authenticity of these overprints on the basis of a scan.
I would weigh in here and say that one cannot determine authenticity on the basis of a scan unless the overprint displays characteristics that make it obvious that it is not the 14 point Casson or bold fonts. Also, it is not always the case that the genuine overprints leave an impression on the gum. Occasionally they do in the case of the O.H.M.S. overprints that were printed on a certain type of paper. However, I have noted that on the horizontal wove paper that most commonly occurs on this issue, those indentations DO NOT occur and the gum side appears flat.
The only tip I can offer here is to look closely at the ink. The genuine overprint ink is jet black and has a shine to it when viewed at an angle in a strong light. However, it does not appear shiny when viewed dead on. On this issue, authenticity would only really be a concern on the rare varieties like the Fishhook G. The other stamps are not worth enough of a premium for most forgers to bother with, except possibly in used condition.
That covers all of the official stamps that were issued in connection with this series. Tomorrow's post will cover the proof material that is currently known in connection with this issue, and then on Thursday, I will write about the postal history and postal stationery. That will conclude my detailed posts on the 1953-1967 Karsh and Heritage Definitive issue.