'Six World Champions on a Single Envelope'

For this edition of Top eBay Chess Items by Price, I had plenty to choose from, but they were all autographs. Here is the short list down to my cutoff point, with final prices in US$:-

• Howard Staunton Autograph Signed Letter Great Britain Halifax Chess Club 1842; $3,175.00; 5 bids
  
  
• Hotel Doelen Dinner Menu Chess Tournament Autographs Alekhine Rubinstein 1921; $2,995.95; 1 bid
  
  
• Chess 1904 Lasker, Chigorin, Schlechter, Marshall, Fox signed, Cambridge Springs; $1,499.00; 1 bid • [signed 'Michael Tschigorin']
  
  
• Chess: Jose Raul Capablanca Signed 1924 Original Autograph; $999.00; 1 bid
  
  
• Staunton Centenary Chess Tournament Booklet England Autographed by Players 1951; $499.95; 1 bid
  
  
• Chess match USSR vs. World 1970 signed Tal, Botvinnik, Euwe, Smyslov, Spassky + 10; US $499.00; 1 bid

Which item to choose as an illustration? The dinner menu was visually interesting, but even more interesting was the cover from the 1970 USSR vs. World match, shown below.

The description said,

Commemorative envelope with a special cancel from the famous first chess match USSR vs. Rest of the World held in Beograd, Yugoslavia, in 1970. Signatures of 15 participants - 10 from the USSR team + 2 reserve players (Bronstein and Stein), 2 players of the world team and Max Euwe as the world team captain. SIX (!) world champions on a single envelope!

It went on to list the 15 signatures corresponding to the circled numbers on the cover.

(1) Boris SPASSKY, (2) Fridrik ÓLAFSSON, (3) Max EUWE, (4) Mark TAIMANOV, (5) Efim GELLER, (6) Mikhail BOTVINNIK, (7) Paul KERES, (8) Tigran PETROSIAN, (9) Vlastimil HORT, (10) Leonid STEIN, (11) Vassily SMYSLOV, (12) David BRONSTEIN, (13) Lev POLUGAEVSKY, (14) Viktor KORCHNOI, (15) Mikhail TAL.

Six world champions on a single envelope -- is that a record?