yeah, could very well be a 55c. Its missing the top plate though. Looking closer, I see where the knurled nuts are to hold the plate. Mine was also missing when I got it, but I got one recently off of a parts chassis for the grand sum of $0.
I believe its a field coil speaker too, so you'd need to fake that out. From fuzzy memory here, the output transformer also lives on the speaker, and the 55c is push-pull 45 output tubes. I have a 55c in a highboy cabinet, but its been a while since I worked on it. I do remember the original speaker has a leather surround, which I thought was interesting.