"Bis" means two (or twice) in Latin, and "ter", three.
In this case my catalogs show that many grades could be ordered with an extra quality for 10FF more. There is a blurb early in the catalog, explaining those are just better than average guns giving better results for range and accuracy. Looks pretty arbitrary to me, and a marketing ploy to up-sell, no doubt.
I did not see a "ter" in my 1900 and 1910 catalogs but undoubtedly it would be a similar thing.
I have never seen an ideal marked "bis" or "ter" so there is probably no way to know now what the difference was...
Best regards,
WC-