I've been waiting for someone with a little more knowledgeable to chime in, but since they haven't, I'll give you my $.02
It's been suggested by those in the know that the earliest 45-70 No. Ones showed up in 1970, even though they may have non-prefix receivers (both are possible, but both are considered "non-prefix" guns).
Of course, these earliest examples have the 1st variation grip checkering and may have either the typical non-prefix fore-end checkering pattern, or the post-1969/70 pattern that continues up to today.
From somewhere around serial number 130-04999 to sometime in 1976, the grip checkering was the 2nd variation that your rifle should have.
I had one of the first 45-70s and sold it too cheaply, like most of the other No. Ones that I let slip through my fingers.