The term “Military Censor” is a poor description of what should be called “Military Intelligence Review”. I say this in that a military censor (usually a junior officer) is one who censors mail to/from military members. These are usually at the unit level and check only mail of their particular unit.
The role of military intelligence (MI) is to find out about opponents, and to find out what the enemy knows about us. The censors role, on the other hand, are to prevent the enemy from finding out about us through emissions from ourselves. One of the tools that MI uses is mail surveillance. The two agencies compliment each other, but MI has a much broader scope, and encompasses many different subjects and players, not only just the enemy. Since Jamaica was a place for various groups plotting overthrowing governments in the area, Cuba, D.R., Honduras, etc, monitoring their activities was important to the war effort.
The JCH1 and JCH3 tell us MI looked at a letter, but not what they did with it. They may have just checked against various lists or a full-blown test. They also did random samples of the mail stream, thus the appearance on civilian mail, American contractors, military mail, and transit foreign mail is not unexpected. What we don’t see is when they review consular mail or mail where it may be advantageous to not advertise it is being reviewed. DEFE 1/160 (Mr Pilkington’s Visit Report of March 6-13, 1943) speaks of cross-fed between MI and Censorship, in particular consular mail. I doubt they were helping censorship, per se, since I think they had plenty on their plate already.
The second question re the manuscript marking on the PC90's is still up in the air. We’ve seen only 1, 8, 18, 19, and 28 in manuscript. (8832 and 8839 were done in manuscript on JL80, but believe that is unrelated.) I think 1 is MI, while rest probably are Jamaican censorship. They have only appeared on type JL54 (Burrows L8) which was used 3 Sep 42 to 5 Sep 43. None on the other labels requiring number insertion, i.e., JL56, 62 and 80, nor any on labels used in the Caymans or Turks. Why only these few? There really aren’t enough for a good sample. My guess on MI is that JCH1 stamps were on their last legs (they were WWI stamps), and perhaps was broken/lost in early 1942 but the JCH3 was already on order, so no sense in making a replacement. The other ones are less clear, although my guess is that it was close to the transition time from the Jamaican Censor Service to the Imperial Censor Service. Note that both are guesses - we really don’t know.