Re: What is this?
Not sure of the exact make/model, but there have been a couple different outfits that make retractable ballistics chart. I know that Leupold make them under the Mk4 label in both 1" and 30mm varieties for a while and they work basically as you described (I can't recall who made them for Leupy). Tape measure type setup that extends/retracts giving you the most basic ballistic info for your rifle. The Leupy version has been discontinued, but you may be able to track one down if you'd like to try one or find them by searching for "Retractable Ballistic Chart" on google, etc.
