Poor resolution but Superjet removed the Tweet soon after so I had to rely on a screenshot found on airliners.net!
Seems to be K, N, W or X as the middle-letter
By a process of elimination:
EI-FKA used to be on an Aer Lingus F50
EI-FNA is registered to the latest Stobart AT72
EI-FXA is on an ASL AT72
EI-FWA doesn't seem to have been issued
So the latter?
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