Collecting old postage stamps from Bulgaria from the period from 1881 to 1931 is a fascinating hobby! During this time period, there were several heads of state depicted on the stamps, each with their own historical significance. Here are some of the main figures you might encounter:

1. **Alexander I of Bulgaria (1881-1886)**: He was the first monarch of independent Bulgaria after Ottoman rule. His image could appear on postage stamps from the early part of this period.

2. **Ferdinand I of Bulgaria (1887-1918)**: He succeeded Alexander and ruled during a turbulent period in Bulgarian history, including the Balkan Wars and World War I.

3. **Boris III of Bulgaria (1918-1943)**: The son of Ferdinand I, he was king during the 1930s and the beginning of World War II. His image would also be found on postage stamps from this period.

4. **Ferdinand I and Boris III together**: There may be stamps depicting both Ferdinand I and Boris III, especially in the transition period from Ferdinand to Boris's reign.

5. **Political figures and symbols**: In addition to the heads of state, there may also be stamps depicting political figures, national symbols, landscapes and historical events of the time.

It is worth looking for stamps with these images and studying them for their historical and artistic value. You could find them at specialized stamp shops, auctions or online collecting platforms.