The HP19bII hits a sweet spot in me. It was the calculator that was recommended when I took my MBA after engineering school. (The HP17bII was the other, cheaper alternative). It had everything the HP17bII had, plus 3 lines on screen and a more populated scientific menu: it included everything an engineer could need, short of matrix algebra: several conversion types, probability, trigonometrics and hyperbolics, etc.
The problem of the breed (common to the other clamshell units, the HP28 family) was the design of the battery door. The batteries placed a lot of stress on the door, and this force was supported by a limited area. When the calculator fell on the floor, the door usually broke down. Also, the plastic used was quite brittle compared with other models, making it more fragile over time.
At some point in time, there was a redesign to bring the battery door from the side to the back, solving the universal problem of the side door. In my opinion, also the plastic is a different formulation, softer and more flexible. Looks darker, too. The keys are black instead of grey:
And here you can see the back of the calculators (anf the different plastic color):
We have solde several units, but this one is for me! (By the way: I received one without the door, so we designed a 3D-printed door for the back door HP19bII - please contact email@example.com for info if you want!