Storing by entry number would require a mapping from entry number to position, or (trivial mapping) with all entries being maximal size.
How odd. I would expect that, in practice, older belt items would leave the output latches for some space in the spiller – entries that haven’t fallen off yet and need to still be kept around, but have still been around for a few cycles; or things spilled back in after some nested function calls. I expected that an entry-based Scratchpad could simply be an extension of those locations.
But this is likely just gut-feeling not matching reality.