Because I got the question I had to do some digging in the archive, As it turns out I found the starting month back in one of my own publications that I wrote in 1990:
This booklet was published in 1990 to document my first 10 years in Mail-Art. I still have the originals here. In it I wrote about the start of the IUOMA. It turns out to be AUGUST 1988. Later in 2005 it is also documented in the book "25 years in Mail-Art).
It started as a joke, but survived for 25 years soon.....
I will do some more research soon (I keep track of things quite well) and will read notes I made back in 1988 to get the starting moment back in details.These are private notes that are archived in the TAM-Archive.