When I started with the CSP in 1981, the S&W 66, in a "blued" finish, with 125 grain Remington SJHP's was issue. As soon as the 586 hit the market, I purchased one of the first ones off the assembly line, and carried it until we were forced to switch to the Beretta 92fs in about 1985.
Issue shotgun was the Remington 870, with Colt M-16's available in the troop armory for "special occasions".
Back up guns were your choice, as long as they passed the safety inspection conducted by the range Sgt. Walther 380's were popular. I carried one in my boot, along with a four shot mini 22 mag revolver in my pocket.
When I retired from my fed LE job in 2009, we were carrying SIG 40's, along with issue MP-5's, and M4 carbines. The 870 with #4 buck was still the shotgun of choice.
Larry