The isobirl formation is due to Terry Glasspool. I (Rod Downey) learned of this formation from my correspondences with Malcolm Brown.
Rod also points out that according to Murrough Landon, the dance can easily be adapted as a 6-couple dance: If 2nd & 3rd and also 4th & 5th places do the Link on bars 39-40 then the progression 241635 works. (Anselm adds: For that matter, it would also work as an 8-couple, 10-couple, … dance, but at some point this stops being useful. Or do it as a round-the-room dance with two closed circles of couples facing one another?)
You can also turn this into a 2-couple dance in a 3-couple or 4-couple set by replacing the final “link” with another “set” for the two couples. This may come in useful if you end up with 7 couples, so you can have a 4-couple set doing the dance “properly” and a 3-couple set doing the 2-couple version instead of sitting out. If you have only 5 couples, the 5th couple can dance the final “set and link” with the couple in 4th place, and the 24153 progression will work, too.