ENTER↑ is a misnomer, long live ENTER:

[John R]

(07-07-2014 02:54 AM)Katie Wasserman Wrote:  

Quote: But here the HP-35 designers overshot the mark—instead of just having ENTER^ terminate digit entry, they (who knows why?) made it also carry out the unrelated task of duplicating the number into the Y-register, then disabling stack lift.

I don't really know the answer to this, but being an original HP-35 user I thought (and still think) it was to make squaring easier.

Another (related) intention may have been to simplify stack preparation for constant arithmetic -- that is, operations subjecting the X register to repeated addition to, or multiplication by, the same constant value. The traditional four-level RPN stack is prepped for this by keying in the constant value, followed by ENTER ENTER ENTER, followed by the X value. If ENTER served only to terminate digit entry without duplicating stack entries, then the user would be faced with the tedious and error-prone process of keying in the same constant value three times in succession.

In a parallel universe, HP might have used ENTER solely for digit termination, but also provided a separate key (DUP?) for stack entry duplication -- in which case we would presumably now be discussing the HP-36 instead of the HP-35.