Long pepper

All my reference sources state that there are two species in the Piper genus called long pepper:  Piper longum (Indian long pepper) and Piper retrofractum (Indonesian long pepper).  I've never come across any description of the difference.  I've read that they are used interchangeably.   

I've bought long pepper seven times now.  Twice, the pepper spikes were smaller and darker.  The other batches were a lighter brown and about a third larger.  I bought some at Stone Cottage, and it was of the smaller, darker variety.  The kind sold at The Meadow is of the larger, lighter kind.

Does anyone know if these are separate species?  Does anyone have a references detailing the difference?