Since your 640-1 wasn't introduced until 1996 it would definitely have the standard .312" smooth combat trigger. And yours is most likely the MIM trigger with the hollow back.
1986 - New forged smooth combat trigger became standard: .312" J, .312" - .320" K, L & N, including the later MIM triggers, and in 2003 the X frame came with the MIM .340" trigger.
Before that small frame triggers were grooved since the early 1950's.