Farrell bases used to have 1/2 round cross-slots vs. the squared off type. Could be that Ken has since changed the design. You should call KF and ask.
If I had the half-round slot type base, I'd use rings with round cross-bolts, examples being KF's own (but heavy and pricey) and the excellent Burris Signature Zee type (with the plastic inserts).
If your base has the squared slots, then I'd use rings with the squared cross-bolt, like Badger, etc. An excellent aluminum alternative to the expensive steel products is the TPS ring (as sold by Brownell's, and which you can see at the link to follow).