Greetings Paul,
The MARK XVI was my first IWC and I enjoy it today as much as the day I bought it. As previously mentioned by Mark on it's accuracy, I too can confirm that it is the most accurate piece in my collection. The movement is a IWC modified ETA 2892-A2 which is often referred to as a "workhorse".
From your pic's, the watch looks authentic and the style of the strap appears to be OEM but since you bought it used, perhaps a after-market fresh strap was fitted to make it market ready. BTW, did you purchase this from an individual or a AD that also sells estate/pre-owned watches? Of course, once opened and verified, you will then know it's authenticity. If it turns out to be a replica, at least you can say it's a good one as far as replica's go, but no substitute for the real deal.
Follow-up with us on how things turn out. Always nice to hear the outcomes.
Best,
Andy