OK, I think the key thing in the Rival was the wide core in the middle with the tapered top and bottom, very similar to the diamond cores from Lane #1. Great balls on the league conditions of the times.
Another core shape that reminds me of the Rival Core is the balls in the Revolver series from Brunswick. I have the Loaded Revolver sort of pearl, though I love the shape of the core, it is really a slow response coverstock that responds too slow in my house. On a highly reactive properly stripped or super stripped back end house I used it and if I had used it earlier I would have been going to Vegas to bowl for $50K a year or two ago! I have seen others with better reaction on this ball either sanded or pin down. I have done neither!
The Lethal Revolver solid might be better but I have not seen it live.
If your house has the typical wetter oils of today with a typical amount of carrydown I think a faster response coverstock as recommended for the Disorder or the Smack Down might be a great answer! The Disorder has the closer numbers. Lower RG and less Diff than the Smackdown.
REgards,
Luckylefty
PS I have two Blurs arriving at my house today! WooWoo!