Right off the bat:
-The dial is definitely not a Mark VI. I think it's a Mark V (baton markers with black inserts)
-The hands are too light, the closest-color hands on an actual Siffert would be those from a Mark VI but these are lighter still.
-The blue markings on the dial look too light in color as well for a Mark V and I would say they're repainted.
-Should have a tachy bezel instead of the MH bezel as shown; but some Sifferts are known to have been special-ordered originally with MH bezels
-Based on the watch I'm wearing at the moment (a contemporary 1163 V) the serial numbers look to be in the wrong "font" but I'd need to compare to my own (a Mk IV) to be sure

All told, given the overall condition, I'd stay away unless the price holds below ~4,500 or so - as the cost to bring it up to original spec would be high.