WAYNE - I could make a S.W.A.G. at a reason. I have no idea if there were any production records kept of 129's with hydraulics. Paul Bell may have access to LVL records like that.
My personal opinion would be that IH LVL did not keep track of that type of info. Hyd's on a 129 could be either factory or dealer installed. Also a chance a kit may have been made available to customers as well. I don't think hyd. was part of the serial number so no way to track actual production on 129's since the exact same rearend, hydro, plumbing, & valves were used on 149's & 169's. It was an option that should appear on factory price lists, and the kit appear possibly on "finished goods" or "Attachments" price lists.
From having spent time @ IH ag dealers in the 1960's & early 1970's chasing parts mostly, Dealers only kept a couple new CC's on the sales floor, 129's were REALLY popular, the hydro had proven their durability and productivity in almost ALL uses, a 12 HP hydro could run anything, but to be competitive price-wise with the little green tractors, red, orange, and other colors, I suspect most CC's were ordered by the dealers with minimal options to hold down the price. Compared to combines, tractors from 60 to 125 HP, big tillage equipment, a CC was a small purchase or sale for most dealers. Most customers bought what the dealer had available when they needed something. And most customers (predominantly farmers) bought CC's for mowing. Urban & suburban customers would get snow blades or blowers but all farmers had bigger equipment for snow removal.
Most dealers in the midwest relied on big equipment sales and service work for profit. The L&G equipment was a very small percentage of their business. As the model mix became broader in the 80's & 90's most dealers would keep an SGT and 3-5 smaller CC's on the floor. My one local dealer even has a separate show room for his L&G stuff and ATV sales.
Funny but true story, Good friend of my Dad had a pair of BIG green mowing tractors, one with power steering, the other without P/S. Guy's Wife compained one day that she NEVER got to run the tractor with P/S. Guy shut his tractor off, got in his pickup and left. Hour later he returns, ten minutes later the dealer's truck pulls in the yard with the newer model green mower WITH power steering. The dealers trailer was empty when it left the yard. Most mower buying decisions weren't made THAT quickly, but it happened!