Okay, I found my Sellers genetics info. Rate of feathering is controlled by a sex-linked (not autosomal) gene symbolized as K. There are four possibilities at this allele. Small k designates the recessive gene that results in rapid feathering. Capital K
designates late feathering. K>s results in slower feathering than K, and K>n indicates Very slow feathering. This one results in bare patches on the backs of juvenile birds, especially males that are K>n, K>n (have two copies). The good news is that since rapid feathering (k) is recessive - once you have it in the pure form it will always be there and will not be lost. Desireable recessives are the easiests genes to segregate and retain.
Incidentally, the symbol for dominant rumplessness is Rp.
There is also recessive rumplessness symbolized as rp-2, and it is associated with a skeletal deformity called roachback.