"That said the base on the LRH appears to be a custom job. Quite likely made to spec for Savage by EGW."

That's what I was thinking.

They also make a steel HD base too but I think they only make it for short action Savages the last time I checked but it's been a while since I've looked. I have both the standard and the HD and you're right about the types of aluminum on them. The standard is 6061 and the HD is 7075 which is harder/stronger.