In the book, Genetics of Chicken Colours-The Basics says on page 181; Brassy Back is:
e+/e+ Ar+/Ar+ Di/Di Ml/Ml "rb/rb'
e+ is the wildtype expression of the e-allele
( the canvas on which the other colors are painted)
Ar+ is autosomal red (not sex-linked red/gold)
Di is Dilute gene (diluter of red and gold)
Ml is melanotic gene (extender of black)
rb is recessive black gene. (extender of black)
The e+ locus and all these genes are discussed in various threads here on BYC.
The book also says " in the English countries, it's a colour of Old English Game and is very variable in expression." end quote
karen in western PA, USA