China: How To Identify Originals/Reprints S1 National Emblem

The colourful S1-set, National Emblem, is one of those early Chinese issues with reprints that are very hard to identify. The differences are very minute: the paper of reprints is said to be thinner, the colour of the stamps should be darker with the reprints. But those differences do matter, for originals have a value of some 50 dollar mint/25 dollar used, and reprints just some 3 dollars mint or used. A set of originals with a numbered tab can get you 180 dollar...
I only have one original, an 800 stamp with smudgy cancellation and a horrible backside (proof that it really was attached to a cover). And yes, the paper is somewhat different, but is this a reliable detail? Not really. Is it really lighter in colour? Not really. There must be other differences... Well, all the details on the reprint are a bit smudgy and much cleaner on the original. That really will not give you a clue when looking at pictures of sellers, though.
Used stamps will have the light CTO-cancellation if they are reprints, and big, black, smudgy ones when original. That's easy. But how to buy mint originals with confidence? I really would not know. Do you?


A complete set of the nice Emblems (reprints, I presume...)


A reprint and an original. The original is in terrible state, a sure sign it's a used original!


Look at the backside of the original stamp at the right! Maybe I should try to soak it in water to get rid of all that glue and mess, but it's my only original, so will I dare risk it?

This is the exact description from the official catalogue (by Posts & Telecom Press):
The first print is on both scripted and unscripted paper. The reprint has thinner and whiter paper, the back side is comparatively rough, and the ink is lighter than that of the first print.

So the repints have lighter colour, not darker as you said.

This is the most difficult one.