This issue is a bit different from the other 2 emissions.
First of all, it's only one stamp this time, not 7 as in the first and second emission.
Secondly, the stamp has a 'high' face value of 100 baht. (the other stamps only had a face value of 5 baht each).
The stamp however remains heptagonal, as a symbol of the 7th cycle (7 x 12).
The face value is higher, but that is easy to explain...
|100 baht stamp - his Majesty King Rama IX|
The theme of the third series of these commemorative stamps is "Wishing the King greatest happiness". The series come in only one design - a picture of His Majesty the King in royal ceremonial attire. The sheets have 4 identical stamps.
|full sheet of 4 identical stamps|
These are - so far - the only heptagonal stamps that I know of.