The standard siffert autavia was produced (by default) with the tachymetre bezel. However, it is likely that you could order a different bezel at point of sale, or at very least at a later service.

The deci bezel, as on your watch, was definitely produced by Heuer. Here is a deci bezel on a viceroy of mine (the bezel was added later - as the viceroy was an economy model I doubt you could order a custom bezel).

In case there are sceptics out there about the deci bezel, here is a picture from the recent collector summit at the TAG Heuer factory, a tray of bezels exhibited as part of what TAG-Heuer termed their "archive":

It is not really possible to prove if the bezel was ordered as a custom option or was put on at a later service, but from what I can see the watch looks quite original (hands, dial look untouched) so I'd guess the bezel was fitted orginally as well.

So enjoy your siffert, very nice catch!