I too feel that its a little too thick to be the 500 f/4 and rather its either the 400 f/2.8 (with a 2x TC) or the 600 f/4.
Why the 400 f/2.8 with a 2x TC?
- for the same reasons already mentioned plus the fact that the portion that attaches to the mount seems rather long.
Why the 600 f/4?
- the 600 f/4 does have a rather long portion just before the mount
- the closeness of the red line could just be due to parallex errors (i.e. it may actually be further than what we are seeing from this angle)