The history of Infinity is complex, and since the brand is now owned by Harman, this comment is not to be construed as "definitive" and I should defer to them.
The Infinity IRS Beta is a different animal - it's not a line source as it has a ribbon tweeter, ribbon midrange, two ribbon mid-bass and woofer towers. The RS1B though, has a line source ribbon midrange array, and three ribbon tweeters. So, it is more like the Genesis II.5, but as it is a 4-tower design, it would be somewhere between a short G II.5 and a short G II.
The G II.5 developed into the G300, then G350, then G350SE.
I tried to rationalize the Genesis range into:
1) Genesis 1 : this is the ultimate cost-be-damned versions, line source, 4-tower with huge wide wings. You need to live in a castle or at least a mansion. I would put the Infinity IRS in this category.
2) Genesis 2 : this is the first level down, line source and smaller rooms. Current model is the G2.2 (4-tower) and G2Jr (2-tower). The Infinities in this series have the tag "beta" as in IRS Beta, RS1B (shorter but line source), etc.
3) Genesis 3 : still a twinkle in my eye
4) Genesis 4 : no more a line-source (because of size/height constraints). The current G4 has a ribbon tweeter, ribbon midrange and dynamic mid-bass couplers. The old Infinity equivalent would have been a 2-tower version of the IRS Beta, or the IRS Gamma.