[math-fun] Origin of SQRT hack?

When making an initial guess of sqrt(x) for a Newton iteration, where x is a positive floating point number, a good initial guess is the bits of the entire number (both exponent & mantissa) shifted right by 1. This is the sort of thing that would have been done on the 7090 or the PDP-10. Does anyone here recall seeing this trick in the 1950's or 1960's ? I checked, and it isn't in HAKMEM. I've used this trick myself (at least on machines where moving numbers between the integer & floating point units isn't prohibitive).