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.
toebert
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.