Triggers wider than .265" are flared on the sides to fit in the frame.
A few milestone dates regarding grooved triggers:
• Serrated trigger change order March 18, 1929 by F. H. Wesson on all .32-20, .38, and.44 Hand Ejectors, .265" wide. Change trigger pull on all HE Models to 3 ½-5 ½ lbs ordered May 1, 1929. So in 1931 I believe the K22 began life with serrated triggers.
• 1955 - Target trigger N (.500") grooved introduced early 1955 until between 1980-99. Much later than 1955 the .400" grooved and smooth target was introduced. The .320" smooth combat trigger became standard ~1986 and all optional target triggers dropped soon after.
1986 - New smooth forged combat trigger became standard: .312" J, .312" - .320" K, L & N, including MIM triggers, and in 2003 MIM .340" X frame trigger.
The only optional .312: grooved trigger was for the J frame. I've never seen or been able to confirm a factory smooth let alone grooved .312" combat trigger before 1986 for a K frame.