ENTER↑ is a misnomer, long live ENTER:

[Thomas Klemm]

(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.

According to The Evolution of RPN & Numeric Entry the ↑ key predates ENTER: it did the same as DUP. Entering a number would always overwrite the X register of the stack. The Y register was the accumulator displaying the result of the arithmetic operation.
But the HP-35 has only one line to display. Of course you want to see the result of each calculation. Thus the X register is dropped. But now for chained calculations you had to press two keys for each operation. To calculate 1 + 2 + 3 + 4 you had to key in:
1 ↑ 2 + ↑ 3 + ↑ 4 +
I could imagine that this was considered suboptimal. Instead of overwriting the X register the stack could be lifted when the first digit was entered. However this lifting had to be disabled for the digits that followed.
What about CLEAR x? To keep the same behavior as before entering a number should overwrite the X register: thus lifting the stack had to be disabled.
We still need the first occurrence of ↑ between 1 and 2. Here as well lifting the stack has to be disabled. Otherwise we loose the Z register.

Of course this is just a wild guess.

Cheers
Thomas