: The reference number is 773303 (never heard of) and the serial
: number is 215342.

Unfortunately, there are a couple of problems with that.

While Heuer did move to six digit references, I don't have any record of any watch before serial 266xxx having such a reference. Around the 215xxx mark, Autavias were using four digit references for automatics (i.e. 1163) and five for manuals (e.g. 73363).

I don't see a six digit reference Autavia until 297xxx, with the 7741-driven GMTs and Shauntavias, ref 741603.

And that raises a further problem with this reference. The first three digits (e.g. 773 in your case) reference the movement. So far so good.

The ending -3 denotes a steel watch. Again, we're fine up to this point.

The fourth digit, however, represents the model and it's here we have an issue. The Autavia is denoted by a 6. 3 is a Monaco and we're obviously not dealing with a Monaco case here.

When the watch is in your hands, please check that reference carefully to see if it's not in fact a 6. That would be interesting.