What is an iconic Shark mech for the period between the invasion and the jihad?
Tricky-ish question. The Sharks were big on selling to everybody and anybody, and likewise, because they had no real major enemies within the Clans (even the Snow Ravens, who they weren't on fantastic terms with, they still dealt with), were able to get pretty much anything they needed. This means that it is plausible for a Diamond Shark to be piloting almost, and that things that would potentially be iconic Diamond Shark Mechs tended to propagate to everyone VERY quickly.
As I recall, they were a heavy user of the
Mad Dog, which is perhaps the most ubiquitous Clan Heavy. They wound up as producers of the
Warhammer IIC, the
Phoenix Hawk IIC (thanks to some wheeling and dealing with the Vipers), and at least one of the other IICs that escapes me at the moment. They produced the
Piranha, the
Mad Cat II, the
Thresher, and the
Ha Otoko as well, and I wouldn't be shocked if they had produced other stuff (since Clan Production has always been vague: the Coyotes are mentioned as having a Dire Wolf facility in WoR that hadn't been mentioned in any previous work to the best of my knowledge).
If you're looking for iconic Diamond Shark Omnis, the one that immediately comes to mind is the
Grendel/Mongrel. And that one was sold to pretty much everyone.
Since my brother has most of the TROs that I enjoyed in my childhood, I feel I am forgetting some iconic Shark Mech somewhere.