Not that familiar with Calculators but the MH on an 11633 MHT refers to the outermost bezel having a minutes/hours scale on it:

Now, I've only ever seen the following type of bezel on a 110.633:

but has anyone seen it with a minutes/hours bezel? That would neatly solve the question, if they are known to exist! The other thing that occurred to me was that most Calculators have a mix of metric, imperial and US imperial measurements on them - I wondered if there were metric only bezels out there, which could also explain the designation. I don't understand why they would do that necessarily, as it wouldn't allow the conversions, but you never know.

In either instance I could understand why the case would be stamped differently, as it's not just a dial swap on a Calculator but a physical part of the case - and a less easily changeable part than an Autavia bezel too. Possible explanations, it needs someone familiar with Calculators to chip in.

298xxx is the start of a block of Calculators that extends into 299xxx. As always, dating isn't an accurate science yet (and possibly never will be) but I'd put that serial as somewhere around 1974.