Chris,
I have a 428 Melling HV
oil pump here on my bench. The spring on the pressure by-pass feels much too stiff to be the spring pictured in your gallery. Here is a pic of the pressure relief valve assembly on the Melling web site (fig 2):
http://www.melling.com/support/bulletins/view.asp?id=10
The assembly does not appear to include a ball.
Keith