The Israelis have one of the most professional & powerful armies in the modern world. One of the highlights of their success is " secrecy ".

If you ask an Israeli serviceman, where he's stationed or what he does, he'll bluntly tell you, that it's none of your business.

Even in postal matters, it's a top secret, where the mail is emanating from, and all outer wrappers ( such as envelopes) are destroyed upon receipt. Outgoing mail is marked by the Army Postal Service with a special purple triangular hand stamp featuring a simple Army logo, an originating number, and the Hebrew letters
" D, TZ " ( for Doar Tzahal = Army Mail ).

So, here we have a real find, in several hundred un-opened letters from soldiers thru-out Israel ( all addressed to a newspaper contest and dated in 1971 ) each featuring the special triangular Army Postal marking, an unusual offering.