SRM PM6 and PM7 rechargeable power meters feature 2 prong charging ports, while the current generation SRM PM9s use the newer 4 prong ports. (PM9s feature the ability to download data power which is the reason for the extra prongs.) SRM has discontinued the 2 prong charging cables, however the good news is the newer 4 prong SRM USB Charger Cable works with all versions of SRM rechargeable power meter.