Looks like you have the older style. I have both service manuals, one from 2008 and another from 2016.
I checked the knuckle arms on both and they are in fact totally different. So you can't use the newer style.
Here is a couple of pictures to confirm that:
2008 Service manual:
2016 Service manual:
Now about why it broke, I might have somewhat of a theory that also happens with mine, which is a 15 series, but the knuckle arm is similar.
There is a bolt on each housing where the knuckle arm attaches, that works as a stop for the steering. On mine, it's not properly adjusted. So on full lock it will hit the stop bolt before the steering cylinder runs out of stroke, thus pulling the knuckle arm a bit. Mine is pulling about 5/32".
Over time, this will eventually break as the cast iron is not really meant to act as a spring.
This reminds me to adjust mine before warranty ends.
Anyway, to source new parts, I would try to contact the Branson dealer in Rome, GA. I saw a post on Facebook where they managed to find a couple of parts for a Branson F3350 which is about 20 years old.
I believe it's called Mathis Trailers and Equipment sales.