With all due respect..... your post tends to confuse things unnecessarily.
The Hy drive system which is employed exclusively on Colt, Case and Ingersoll garden tractors, is entirely different to "hydrostatic"drive systems. While both systems use oil to transfer engine power to the rear wheels, the design of the pumps are vastly different. Hydrostatic drives are a closed system whereas hydraulic drives are an open system. Both have their pro's and con's.
The question was entirely about hydrostatic drive systems. Injecting hydraulic drive into the discussion does little to answer the OP's original query, IMO. Secondly, the Ingersoll tractor company has no relationship to the other company known as Ingersoll-Rand. Yes, there is a similarity in name but that is the extent of it. However, that is a very common mistake that is made all too often.
My only purpose with the above post is to set the record straight so that anyone else reading this thread will not conclude that Ingersoll tractors were ever made by the Ingersoll-Rand corporation.