It'd be a little counterintuitive for F being best and A (nearly) worst.
You mean like how Low technology components are classed as 'A', and high technology components in mechs are classed as 'F'?
Or how Terra had several categories as 'AA' because 'A' was not high enough to describe Terra's wealth?
I prefer starting with 'A' reflecting near-zero, because it allows for lots of other letters to be used for better capabilities.
For the list, I'd be tempted to add the following to E, C, and A:
E: No confirmation of internal production (suspect that a factory can make it) or possess large internally controlled stockpiles
C: ... or large stockpiles
A: ... or large stockpiles
Would you be extending this chart to components, then using the hardest to obtain item as the Mech's overall availability? I.e. if a nation produced all but the PPCs for a Marauder (so for everything else it was Availability F), but a unreliable power produced PPCs in a factory, the PPCs would have an availability of B, meaning that Mech design would have an availability of B.