Later calculators (the HP 42 and 48 lines) would basically accommodate either mode of operation. You can perform a calculation immediately without pressing ENTER to put it on the stack, or you could put it on the stack with ENTER first and it'll still do the same thing when you press an operator key.

I use an HP 48G+ frequently and I pretty much do both styles without any obvious rhyme or reason from my viewpoint. I just sometimes do one or the other.

I think you mean 28, not 42 — the 42 is also "classic" 4-register RPN. And I’m writing this mostly to push for the excellent Free42 emulator, which runs on just about everything (Windows, macOS, Linux, Android, iOS), and also powers the DM42 hardware emulator mentioned above. Only problem with it is that emulated buttons on a touchscreen are nothing like the real thing; I keep missing keystrokes.

I can confirm, the 28C actually shows the 4 (viewable) registries of the stack, numbered on the left as:

4:

3:

2:

1:

So, if you input 4 [ENTER], 3 [ENTER], 2 [ENTER], 1 [ENTER] you have:

4:4

3:3

2:2

1:1

But if you input 4 [ENTER], 3 [ENTER], 2 [ENTER], 1 you have:

3:4

2:3

1:2

1

if you then press +, you get 3 in both cases, which is the sum of the bottom two lines in the display.

Those are either the same as the "old" x and y registers or the sum of last input and the bottom register, same as the old x.

Yes you are right, I did mean that :)