In this case, 45 is caliber, 6 means commander size, and the 7 means decocking. It's the .45 version of the 1076, manufactured for a short time during Smith's "gun of the week" period.
There is actually some variance in the system Smith has used. The first two numbers will designate caliber and capacity: 39 means single stack 9; 59 means double stack 9mm; 45 means single stack .45 (all .45s have this designation since Smith doesn't make a high cap .45; the exception here is the 945 which is a progressive development of the 745/845 competition weapons). 40 causes a problem as it can be a single or double stack, depending on when it was produced.
The second set of letters can mean frame type (stainless/alloy, blue/alloy or stainless being designated as 03, 04 and 06 respectively), or safety design (43 being doubleaction only and 67 standing for decocking).