Were there a lot of MPU6 machines released? When was MPU6 released?
I believe it was around late 2008 - early 2009, A lot of machines that got £35 - £70 upgrades had both MPU5/6 versions while £70 only versions were strictly MPU6. It's Amazing being in the initial wave shortly before release of the £70 Jackpots has MPU5/6 versions, with the likes of Pacman being MPU6 only.
Count Ya Cash is an oddity that it's a £70 Jackpot only but has both a MPU5/6 release! Given it uses the newer "standards" at the time i can only imagine it was a test bed to see how MPU5 coped with it all.