For Trilliums and Surfsides of that era, electric brakes
were an option. From what I have seen, about 50 to 60% were fitted with the electric brakes
. You can check for electric brakes by looking at the backside of the wheel to see if there are brake drums with 2 wires coming out of the backing plate.
The connector type is not always the best way to check as many times someone will change out a connector and not bother with the brake wires, or put on the larger connector for reasons other than brakes.
If the trailer you are looking at does not have electric brakes, but has a 4-5" square flange, with 4 holes in it, on the backside of the axle
behind the hub, you can buy a set of brakes and drums/hubs that will bolt onto the flange. If there is no flange, you will need to replace the axle
to get brakes.