The key doesn’t correspond to capability. It’s about the connector’s physical shape. i.e. which side is the lil notch on. B has a notch on the right, M on the left. B & M have a notch on both sides and therefore fits into either connector, but it is limited to PCIex2/SATA speed (i’d guess this won’t matter to you as pciex2 is around 4gbps even on pci1). It also doesn’t mean the length would fit, that’ll be a number like 2230, 2280 etc.
You’d have to check the manual for the motherboard to see if it specifies, but if it’s labelled m.2 PCIe it should work I think. If it only said SATA then it would not work.
You can try dmidecode --type slot on Linux, it should list them as available and presumably specify the type to be PCI express.
I think ultimately you’ll only know when you try, but these may give you some confidence.
I believe the fact that it comes up as a “slot” is positive. SATA m.2 connectors don’t show up under slots, they come under "connector"s with type SATA.