GB Machin Phosphor Question

Hi, so using my new shortwave uv light I came across this, I believe its sg x924l strip which is the readers digest strip? , under uv its meant to have a horizontal phospor stripe which is missing from the front but apparently on the reverse, its clear to see under normal light, anyone have any thoughts, thanks

Ps. Photos taken from my phone.

Yes, it's the RD strip from 30 December 1981. It may be the FBO variety ("fluorescent brightener omitted").

SG Specialised Catalogue says of the uncommon strips with fluorescent brightener omitted from the paper, "The strips were usually mounted by waxed strips on Reader's Digest promotional cards. The gummed sides show disturbance to some degree."

I believe what you're seeing may be the FBO strip with "disturbance" to the gum caused by the waxed strips. If so, the paper should have the fluorescent brightener omitted, and that would be best checked with long-wave UV light. The paper ought to react no brighter than a pre-decimal Machin.

If genuine FBO, it's quite uncommon.

1. The line is the adhesive used to affix the strip to the card sent out.
2. The phosphor bars on Machins have a vertical orientation. There is one bar on second-class stamps, either at centre or at one side. There are two bars on all other stamps. For a period, those came with phosphorised coating and not with bars.
3. The phosphor bars show an afterglow. The bars do not necessarily show when you flood the stamps of that period with a uv light.
4. The uneven "whiteness" of the paper suggests either environmental damage to the coating, or incorrect use of the UV lamp.

All this, however, is irrelevant. Your multi-value Reader's Digest strip was never issued with phosphor bars.

And you cannot use short-wave uv light to determine FBO. You need long-wave uv light. If this is the FBO strip, the paper should show that effect uniformly and not just at the ends.

Thanks for replies, all makes sense, im still getting to grips with the uv lights.