Well done changing your batt. Luv the ESD protection!
The next 2 pics are not mine. They are from "Ultramagnetic" and/or "CFR" from years ago on a other forum if my file naming convention was any good. They offer a decent side-by-side of the 2 sizes.
The "tell" that I use if I can see the s/n plate side is the space above the plate. If it's a full plate height, it's the bigger cal 1511. If there's about half the plate height, it's a cal 1516. YaY!
Enjoying mine today!