I have never seen any factory documentation on ivory or stags..Ruger sold them as an accessory item, and did not ship them installed on new guns.. Records compiled in John Dougan's latest book indicate that Ruger sold just over 6000 pairs of stags and 610 pairs of ivory. There is no way to authenticate them, but there are many aspects of the grips that would indicate whether they are factory, or aftermarket/reproductions.. Things like medallion placement, staking style, fit, bevel, etc all come into play.
To summarize the history lesson...
I believe these grips are factory.