1. Portatifs (14th and 15th centuries): short keyboards containing 20 to 28 pipes; carried by a shoulder strap and played by depressing keys or levers with one hand while operating bellows with the other.
2. Positifs (14th to 16th centuries): small ones were set on stands; larger ones rested on the floor; performer played with two hands while a helper operated the bellows.
3. Organ cases of identified organs (either on the stamp, or identifiable using other sources)
4. Organ cases of unidentified organs (one simply cannot determine where the instrument is or who built it)
5. Rows of pipes not in organ cases (I include here abstract artistic renderings of organ elements)
6. Organ consoles: often showing a well-known organist at the console
7. Features of organ cases (details of pipes or facade elements)
8. Street organs
So in the future I will use this numbering system as an additional label. It will take some time to go back and catch up earlier posts, but I'll get those done also, eventually.
By clicking on any of the labels in the right column you can get a list of stamps related to that particular label. In the case of country names, you get all the posts for stamps issued by that country.