This rather attractive air mail stamp depicting the garuda bird, engraved and recess printed by Waterlow, was first issued January 3, 1925 and continued until 1937 in two sets:

Set (A) of 8: Issued 3 January 1925 . Perf 13 ½ – 15.

2s. purple/yellow (April 21, 1925)
3s. chocolate
5s. green
10s. orange and black
15s. carmine
25s. indigo
50s. black and orange-brown
1 b. sepia and indigo

Set (B) of 6 Issued 1930-7. Perf 12 ½.

2s. purple/yellow
5s. green
10s. orange and black
15s. carmine
25 s. indigo (1925) Note: This issue is found imperforate between vertical pairs.
50s. black and orange-brown (1937)

Stamps of this issue, perf 14-15, with Siamese overprint signifying “Government Museum 2468” were overprinted in connection with a fair of the Government museum, which was to be held in 1925. Owing to the King Vajiravudh’s death, the fair was cancelled and the stamps were not issued for postal purposes.
The stock was used up in 1928 for interdepartmental accounting purposes and some post offices cancelled them with their ordinary date stamps.

This set is worth acquiring. It would make an interesting addition to an Air Post thematic collection, seeking out the various perfs and perhaps a sample air cover to complement them… Good luck!