I and several computers are increasingly preoccupied surveying similar "growth shoulder" mechanisms found across some but far from all rules of the form WMPVN-45678x/459/n, where x indicates the presence or absence of c (12).
It is still too early to try to report anything comprehensive, but the rule which scores highest in the intersection of productivity and interest is 45678c/459/11 where I've been running viable seeds to 70,000 iterations, beyond which they become impractically slow, and using full screen capture at 24" iMac resolution to record mostly still growing structures which are uploaded to:http://wmpvn.com/45678xx_45xxxx/c_9_11/images/
The currently 50 PNGs cover 46 developing structures, the three biggest as left-right pairs and one ancillary flotilla, with more on the way.
Having proclaimed in a conference paper in February that attractor basins are it, this rule has since shown that escape from chaos into some periodic process is not everywhere guaranteed. While there is one relatively very short period attractor and at least one very long period one in these examples and others suggest aperiodic shoulder growth can remain at risk of termination, more than a few show no obvious indicator of a likely end state. Further reflection and some less productive recent work with /7 suggest uncertainty at the edge of chaos is something that should have still been admitted, but had become obscured in February by a long run of emergent orderly forms under other rules in the family.