It's an M91TS upgraded to M91/28 configuration. The small letters you see have nothing to do with Finland - they're inspector marks. Finland bought only M38 short rifles in 7,35 caliber.
You've an uncommon one - few antique M91TS survived the various conflicts. PO is the steel vendor - Poldi in what's now Czech Republic.
The caliber is indeed 6,5. SW