The HP15c Collector's Edition has a HP16c calculator, hidden as an Easter Egg.

To find it, switch the calculator off, press g and ENTER simulteaneously and keep it pressed while you press on. Then release all buttons. You get the selftest menu, with options 1, 2 and 3.

Well, there is a hidden option more: press 4. You come to a screen with three areas: 15, 15.2 and 16. To choose among them, you just need to press the keys closest to them..

• 15 is the standard calculator with 98 registers. What you get when you reset the calculator.
• 15.2 is a non-supported version, with twice as many registers.
• 16c is the secret HP16c version within your HP15c CE calculator.

When choosing HP16c, the calculator memory is reset. Same happens when you move from hp16c to hp15c - any of them. When moving from HP15c to HP15.2 and viceversa, the memory is not reset; however, when moving from 15.2 to 15 you might lose data and programs - depending of your actual use.

Coming back to the HP16c: please see the manual here:

https://literature.hpcalc.org/items/589