I had always heard that IMR7828 was Winchester's choice for the .264 Mangleum. It's one of the VERY few times a canister powder was used for a factory load.
On so-called temperature insensitive powders, there are so many "yes but" and "only if" and "except when" disclaimers attached to them that I treat such advertising claims as essentially meaningless.
I pulled a factory 140 power point a few years ago, it looks like they switched to some sort of ball powder that looked a lot like 780
