Need Help With Cuba Variety

I have 3 copies of Cuba Scott's #364 pictured below.
The top copy appears to be the one described in the catalogue.
The middle copy shows some brownish darkening most noticeable around 'CUBA'. The bottom copy appears mostly brown. My question is, 'Are the middle and bottom copies known varieties?'. I don't have a Cuba specialized catalogue and have been unable to find any references online. Thanks in advance for any assistance.

Looks like they are oxydized - environmental damage

Peter

Yes, definitely oxidized. Since they are not expensive, a short dip in hydrogen peroxide will restore the second and maybe the third stamp to prove what we say.

Sulphurization, oxidation.

http://www.stampsmarter.com/learnin...idation.html

Thanks to all who took the time to reply

I wouldn't bother with the hydrogen peroxide. Note a particularly valuable stamp. I think it makes in interesting study on the stages of oxidation.
Do a little write up.

I decided to try the hydrogen peroxide as an experiment. I soaked the worst example for about 45 minutes. Here is a picture of the results...



The results were pretty astounding. I tried this on a couple of other stamps that had 'yellowed' or 'off color' paper. It didn't seem to do much for those stamps. Are there any tips for improving stamps with discolored paper?

The method you are using only works for stamps that are oxydized. Other discolored stamps ( Like sunlight fading etc. ) do not respond to this treatment.

Peter