Here I see next one issue that is really-really worth to have!
It is talking about King Aleksander Karadjordjevic (1888-1934), named as Aleksander the Unifier, or Aleksander I of Yugoslavia.
It is basic set, issued on 1926 year, with 12 value in set, than, after two year in use, come need to overprint it, and on 7 Dec. 1928, come to the public, set of 10 stamp, overprinted with XXXX , intention is to cancel additional value on the basic set.
Someone will ask me, why exactly this set to have in collection, as we have basic set from 1926 year and it is same catalog value? For simple reason, that this "basic" set have all three combination on it! Basic set, and second one that come in same 1926 year (additional value on basic value), and in the end this one, where overprint are on the additional value overprint, actually canceling this addition value!
As I see in Michel catalog, it is very highly valued for mint, nh condition (900.00 euro), but with lh condition, catalog value go down to 350.00 euro, what I see on internet auction/s around, starting prices for mint, nh go from 200.00 $ to 600.00 $, depending who and from where it is offered, but, I made some "zoom" on same offering and see that both samples are too even more that too fine condition, that give me very high impression that set is or re-gummed, or washed, or some other "manipulation" made on. As same Michel catalog suggest, there is many fake overprints on market available, and everyone who decide to invest in this set, need to made very good examination, or better, to ask for certificate (which will again, rise price for next 100.00 euro per exam).
What I decided when showing this one sample? First, to minimize possibility of fake, I keep one set with examiner mark on back of stamp, there is good reason that who stamped it, know well that it is original, and second, I prefer one set with this "old" glue on it, as it is original gum, used at the time.
Next reason? Simple, it can just go up with price, because no one will sell it in near future!