Thank you for your purchase.
It was originally a RW page type plugin developed by Josh Lockhart many years ago (before Stacks was invented). Then a stack was developed that did the same thing. Under ownership of Jonathan Head both the plugin and stack flavours were available to buy. In recent times, the plugin has been discontinued (it was too expensive to maintain and sales of it were virtually non-existent). Now it is only the stack being developed.
The stack has all the features (and a lot more) of the plugin, so you are not being short changed! The price has been reduced and the licensing relaxed so you can now use it on as many domains as you want.
It seems a lot of people prefer the stack variant, because all the settings are in the “normal place” and you can integrate the stack quickly into existing pages / setups without much trouble.
The stack received a major update earlier in the year with lots of fixes.