For a long time, it was common practice for the resellers of grey market watches on the internet to remove the serial numbers like this, to avoid implicating the authorised distributors who supplied the resellers with the watches in the first place. As the manufacturers generally didn't want their watches sold via the internet, especially at significant discounts, the sellers had to resort to getting them from co-operative ADs and then removing the serial so the manufacturer couldn't easily trace back which AD had been supplying the reseller.

I'd say in the timeframe for your watch, this practice was the norm for a grey market watch. I'm not totally familiar with the Monaco re-issues, but I believe Armando is right - check the case back underneath the model number for any sign of polishing to remove a serial number. They typically have done a pretty good job of obliterating them. So chances are, if there is no serial there and any sign of polishing, then your watch was probably originally supplied via the grey market.

That doesn't mean anything is wrong with it, just that the serial was obliterated to "protect" the supplying AD - some buyers do want the number intact though, if you ever come around to parting with it. The same goes for vintage too - I'd rather have a dented and scratched case with the serial on it, than a pristine repolished one but there are other people for whom that doesn't matter as much.