Thailand is the only country in this region which has never been colonised in its history. When all its neighbours were been colonised either by the British or the French, Siam ( as Thailand was known) remained an independent Kingdom thanks to various treaties signed with the colonial powers. HM King Rama 5 thought that there must be a holy deity which is protecting the country and should be sculpted and worshipped. An 8" statue of Phra Siam Thevathiraj (or City God) made of solid gold was sculpted on his orders and is worshipped till today.
This series of coins, known as the Portrait - Phra Siam Thevathiraj coins (เหรียญทองแดง พระบรมรูป - พระสยามเทวาธิราช ) carries HM King Rama 5's portrait on obverse and Phra Siam Thevathiraj with denomination and date on reverse.
Denominations issued were 1 Solot, 1 Att and 1 Sio. Only the first year carried the C.S. date (CS1249 or 1887). Then onwards the coins carried RS dates. Die rotation varieties exist for all three denominations as given in the checklist. The 1 Att coin for years RS 114 and 122 have different die rotation and date numeral size combination.
Click to see Checklist of Portrait - Phra Siam Thevathiraj coins.
Please refer to my previous posts Thailand: Circulating coin denominations and Thailand: Coin Dating to understand denominations and dates.