There were multiple disagreements on genetics theory by Kimball & geneticists such as Hollander, & later Smyth. Kimball had unusual theories (very hard to follow, not substantiated).

Eg, one of my old posts on Kimball & ER:
Originally Posted By: KazJaps
Kimball had some unusual genetics theories (not uncommon for other researchers to disagree at times).

Eg, Smyth in PB&G, p118:
An interesting gene cluster hypothesis for the E locus was proposed previously by Kimball (1954a, 1954b), but was flawed by incorporation of the nonexistent columbian allele (e) into a multiple allelic series with E and e+.

See the following paper:
Elliot Kimball
Genetics of Birchen Plumage Pattern in the Fowl
Poultry Science (1954) 33 (3): 472-481 doi:10.3382/ps.0330472
Full paper.
Eg, Fig 1 schematics

E || e^+ || ER
Br || bR || BR

B = Black (b = non-black)
R = restricted (r = non-restricted)
*notation like dominant/recessive inheritance

* E allele = Br (black, no restriction)
* e+ allele = bR (no black, restriction)
* ER allele = BR (black, restricted)

It was like he was suggesting 'independent pattern units' of an allele, & maybe nearby clusters.

Also the ER & 'restricted' notation was based on early 1900's genetics research, using blue birds on solid eumelanin base. 'Restricted' was in reference to homozygous phenotype of blue (whitish), had little to do with E locus alleles phaeomelanin/eumelanin distribution.

But, Kimball's test breeding data & observations are useful.

Another old post on Kimball & ER:
Originally Posted By: KazJaps

N (Nigrum) (Davenport, 1909) = E today.

P.s., early ER symbol use ([Lippincott], 1918) was not in reference to ER Birchen, but unrelated notation of blue phenotypes on E allele.

E = "Extension of pigment" (black)
R = "Restriction"

Er Er = black
ER Er = blue
ER ER = splash

So "ER" was in reference to two mutations on different loci.

Much later Kimball did some testbreeding of Birchen & Brown Red phenotypes, & used the symbol ER again. But there were some discrepancies with his theories & therefore "ER" was not formally accepted at the time, but later Smyth et al. formally described the ER Birchen allele.

Another post listing the Lippincott 1918 reference:
Originally Posted By: KazJaps
The following paper is quoted sometimes as the first use of ER (Birchen allele), & that R = restricted:

The Case of the Blue Andalusian
William A. Lippincott
The American Naturalist , Vol. 52, No. 614 (Feb. - Mar., 1918) , pp. 95-115

But it is also quoted as the first use of E - Extended Black (cited by B&H 1965).

Yes, Lippincott used "E" for 'extension of black', & "R" for 'restricted', but R restriction was in reference to the Bl gene - dose dependent restriction (meaning dilution) of black (eumelanin), not E^R birchen/brown red phenotype - an increase in phaeomelanin to E.