PCalc 1.7 (for iPhone)
mjtsai.com/blog/2009/07/19/pcalc-17-for-iphone
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Dr. Drang: If you turn on the HP48 Style RPN setting, PCalc 1.7 does the right thing: it enters the number in the x-register and waits for you to type another number before pu...