The -1 came out in 1972 in the configuration of a 3" square butt heavy barrel version. The -2 came out in 1987 and was a special order for the NYCPD. Both of these were in limited quantities.
The normal production 60's like yours (and mine) remained a no dash until the -3 came out in 1988 with a new yoke retention system.
All credits go to the 3rd edition of the S&W book.

The model 60 guns are really quite interesting in the number of special variations that were produced.

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Patriots Forever!!