It doesn't make any difference if the horse is branded or not, the horse is coming from MT, which is a brand state(like NV) and it is required that you have one. Whether you get pulled over or asked for it at a state line or not it doesn't matter, your supposed to have it. Call me paranoid but after being pulled over and asked for BI's I get one.
I have used registration papers, old brand inspections and bill of sales for proof of ownership whether it is right or wrong, it is truly up to the individual brand inspector. Luckily, like I have said before, we have used all of the brand inspectors in our district at some point or another and a have seen our horses(and none of them had fresh brands ..LOL)
As far as CA is concerned, no it is not a brand state, BUT since you(the OP) came from a brand state you need to have one in the event that they ask for it at a port of entry. We just dealt with this all spring going back and forth from NV and CA. I have been through both the Truckee and Hallelujah Jct. POE several times this year and each time have been called inside to show all my paperwork. Perhaps I look suspicious? LOL!
Anyhow, I am glad the OP got her BI and hope she has a safe trip!
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.