In 1977 and 78 Diamond Back offered it appears just one model the “MX”. In 1979 they debuted the “Pro” an all 4130 bike. In 1981 they would introduce the “Medium Pro”. This advertisement from June of 1981 is the first mention of it I could find so far. Clearly the picture had to be taken in Spring to make that issue. It will be interesting to find out, what was the first batch number for the Medium Pro.
Upon doing some further research. I still cant not find any earlier advertisements for the Medium Pro. However. In the August issue of “Bicycle Motocross Action” it shows Harry Leary’s bike and has specs for it. The frame he rides is listed as a “Long Pro”. So perhaps they were already selling the Medium Pro or testing one. More research is needed.
Thanks to a more layed back seat angle it appears to me the top tube on the Medium Pro was almost as long as the “Large Pro”. The top tube and wheelbase were shorter though and the profile lower. Perfect size for 10-13 crowd.
Diamond Back BMX - The Early days 
any reference on 81 small pro. i have a small pro from the research i have done, with the help of sparkys website as well.
I only have the one 81 ad which shows the Medium Pro and I think has a section on the mini. Check the add out it is a two page ad with all the riders sitting on the bike. I dont think it was called a Small Pro but a Mini.