Yes, the contract could be extended for additional work. In fact, I taught a 5 week module in an undergrad course one semester, and I found that our pre-agreed

Yes, the contract could be extended for additional work. In fact, I taught a 5 week module in an undergrad course one semester, and I found that our pre-agreed time was far too little for me to adequately prepare for the classes (and do office hours etc). So I reached out to my department, and we moved to a different contract, where I tracked my exact hours and I was paid for those hours. However, this was pretty exceptional. In most cases, we got either a 102 hour/semeter contract or a 204 hour/semester contract, with only 90-95% of those hours actually worked (in the Physics dept at least).