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1) Do you have a rule of thumb for post WWI 3c/4c MNH plate blocks?
I assume you mean post-WWII (I have no rule of thumb for WWII and earlier, it varies too much). You should not be paying more than 2x face for the 3c MNH PBs (even that, in my opinion, is overpriced but an artifact of the current wholesale market), and no more than 1.5x face for 4c PBs. That's what I charge (shameless plug). But remember, you cannot post buy/sell on SCF until you get past 50 "quality" posts. Keep in mind, that you can ALWAYS find a cheaper seller.
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2) What online reference is best to get an idea of fair value of a post 1940 mint or plate block specimen ?
5) What is the best online reference to learn the history of a post 1940 issue ?
SCF, of course!

Someone else will have to chime in on these 2.
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3) What are your personal favorite scott#s issued between 1940-1960 ?
I'm terribly biased here -- if I'm not limited to commemoratives, then it has to be the Liberty Series #1030-1059A. I love this series and collect the plate blocks by numbers and position. Been at it for a while and not even close to finished. I also collect the mint sheets, but that's on hold because the current prices are too rich for me.
If it has to be a commemorative, well then of course #1075 FIPEX S/S. Guess what 2 stamps are shown on that S/S!

What about you? And also the other Forum members as well!
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6) Where I can buy glassine envelopes at cheaper price?
Phil had some, not sure if they are still available. Again, you need 50 posts before you are permitted to buy/sell from SCF posts. Here is the link
https://goscf.com/t/5541.
Check with him regarding current availability.
If they are no longer available from Phil, then email me your location (in US? which state?) and I'll try to provide you with some suitable supplies dealers. I do not sell supplies.
I need to get back to work, I mean sleep, err... work?
Please drop by SCF often!

k