Hello Ron,
I thought I would give it a day before responding to see if any of the more versed Ferguson guys would give you a reply. No seeing one, I will share with you my thoughts.
I would have guessed your 1955 TO-35 was originally 6-volt, and positive ground. However, it is getting close to the time when some of the MHF tractors were moving to 12-volt systems, so I can't state that confidently.
I checked Tractordata.com, and they state it was positive ground through 1955, and then went negative ground in 1956. They also state is was 6-volt system. A prior owner could easily have converted it to 12-volt and negative ground even if it didn't start out that way.
Two things I would check: 1. Does it have a 6 volt or 12 volt coil? 2. If it has lights, check to see if they are 6-volt or 12-volt. Those might give you some further clues. However they could also have been converted to 12-volt.
Hopefully a more knowledgable Ferguson person can either confirm my suspicions or correct them for us.
JB