What i might say, maybe true maybe not. Chipman would know for sure.
1. IH Canada Hamilton took it on their own the develop from scratch a "compact farm crawler, not from a wheeled machine.
2. It was farm tractor "RED" and no blade offered.
3. IH Chicago was impressed and ramped development of a small crawler based on the 340 wheeled tractor. It sits way higher than the t4.
4. At sometime when the t4's, 5's and td 5's were still ag red a blade was offered for the red crawlers. This was because the t340 came out right away with blades AND still ag options offered.
5. When a blade was put on the t4's, 5's and td 5's there were extra bracing and reinforcement done to the chassis. I think the pinions were improved and changed.
6. If payline means yellow machines, the above improvements were engineered and made standard to all machines. These would be the "B" series machines.
7. The 500 crawlers replaced the t4's, 5's and td 5's and 340's at the same time and diesel only.
8. Most U C parts will interchange with the 1st series 500's. Maybe some chassis parts too. 500's were built in Canada.