The lanyard ring Service Sixes are mostly (if not all) 4" guns. The 2.75" gun that rugerguy shows is a Speed Six.
Here is a U.S. marked SDA-84L showing the typical lanyard ring stud in between both parts of the SN as Terry T mentioned (note the Service Six marking instead of Police Service Six):
There was a different type of lanyard pin/ring on some Speed Sixes, like was later used on some of the SP101s (French?), that moved the entire serial number under the grip - upside down. This is not my gun, just a pic I saved from a 2005 GunBroker ad (I should have bought the gun, but I was broke at the time):
@chet15 I'm not convinced about the SDA86 being made. If there were 500, they would be as common as the 150 prefix RDA86 (still rare, but I have 3 of those and never saw a SDA86, either in person or a picture of one). As
@WAYNO said, they were listed, but unless they all got shipped offshore for a contract, they remain unicorns.
@Terry T here is a pic to enjoy until you can find a GF92, one each of round and scalloped recoil shield models:
Oh, as to the .380 Rim guns, the later runs in the upper prefix range were for India, they had tons of surplus British .38 S&W ammo to burn. The first run was made without a lanyard ring and destined for Bolivia. Those earlier ones were also marked as just Service Six: