So much has been written about the Siam Tiger Overprints, Scott #B12-30, that the basic stamps themselves have been ignored almost completely. Too bad. Bet you didn't know that they rate as Scout fringe issues all on their own without any overprint. The first and second set, #B12-17 and #B18-23 are, except for the 5 satang value, overprinted on the same stamps.
Six of the seven stamps picture King Vajiravudh, or Rama VI, as do five additional different stamps of the third set, #B24-30. King Vajiravudh shown on these eleven stamps had a passion for Scouting. In Siam he was its founder, patron and driving force as well as the President of the Boy Scout Association.
An interesting philatelic letter, used as an advertisement for a San Francisco envelope manufacturer, circa 1930, explains why you might consider placing an unoverprinted stamp in your collection for 10 cents in case you can't manage the cost of an overprinted set. The King is incorrectly referred to as Rama IV instead of Rama VI.
For more information on determining genuine and fake Siam Scout stamps, see Siam Scout Fund Overprints Forgeries Explained
SOSSI Journal, Volume 26, Number 11-12, November/December 1977