Are there any references for this rose?
It is quite logical for the breeder to name a rose after his wife, but I am wondering if this 'Ko (Schuurman)' might in fact be McGredy's 1978 rose 'Ko's Yellow'. Refer the 1988 reference for 'Ko's Yellow'.

[later edit. the breeder advises that this rose was in fact McGredy's 'Ko's Yellow'.]