ENTER↑ is a misnomer, long live ENTER:

[Katie Wasserman]

(07-06-2014 05:18 PM)hansklav Wrote:  But I read that the HP-18C only used 'System RPL', and I don't know anything about the difference in the behaviour of the ENTER key between 'System RPL' and 'User RPL', so you might be right that the HP-28C was the first with 'Entry RPN'.

That's correct, user RPL was introduced on the 28C. But it's the user ENTER, not system ENTER, that's relevant to your post.