Hi Amer - let me start with saying this is a lovely watch, it would appear to me to be in fantastic condition. I must agree that the bezel does look to be an aftermarket item , particularly if it has a blue finish.
There is certainly some evidence of manual Sifferts being produced with MH dials. I clearly remember seeing a few come up for sale over the years and recall seeing them pictured in this forum earlier.
This link from the OTD Autavia reference shows a 3 register Siffert Autavia 73663 with a MH bezel in place :
http://www.onthedash.com/Guide/_Chronographs/62.Autavia/i.Manual--Later_Style/72.Reference_73663/B.White_and_Black/
Its not evidence enough to conclusively say your 73463 Autavia came factory fitted with a MH bezel but its certainly possible.
Enjoy your watch - Darren
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-: I'll leave to our Autavia gurus ape, AMH and Jeff for more details,
: but the "Siffert with a MH bezel" thread has already
: come up and it was ruled out.
: Definitely it needs to be a tachy one, black base with white font.
: Cheers.
: Fabrizio