Apparently, this is an acronym for reckoning time period instead of using the usual BC-AD or BCE-CE.
The journal I am referring to is archeo sciences. Here's a snippet of the article:

I've looked up "Mauryan Empire" and it appears to be dated at the same time in BCE, making BPE=BCE. But what does the acronym stand for? And why do some journals use this instead of a more familiar acronym?