I just traded my 66-5 today in roughly 98% condition for $350 toward an SA Mil-Spec. I bought my 66 new around '98 or so and yes, the -5 is prelock. The only reason for rarity is that the K-frames are, sadly, no longer produced.
Here's a good thread over on the 1911forum that covers the different configurations with dates and changes:
http://forums.1911forum.com/showthread.php?t=118680
#15 06-30-2005, 11:48 AM
rwmagnus
Model 66 dash info
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66 1970 Introduced at serial K949100
1974 Introduced 2.5” with round butt
66-1 1977 Change of gas ring from yoke to cylinder
1978 Introduce 6” barrel
66-2 1982 Eliminate cylinder counterbore & pinned bbl
slight change in cylinder length to 1.62”
1985 Special production of 3” bbl for Ashland Arms;
5000 manufactured, ALU serial prefix
66-3 1986 New yoke retention system/radus stud package
1988 Special 3” full-lug version, with full length extractor rod
1994 Uncle Mike’s grips on 2.5”
66-4 1994 Change rear sight, drill & tap, Hogue grips,
change extractor.
1995-6 Delete square butt configuration; begin shipment in foam
filled blue boxes
1997 Change thumb piece; ship with Master trigger locks;
change to MIM trigger
66-5 1998 Change in frame design, eliminate cylinder stop stud;
eliminate serrated tangs; change to MIM hammer with
floating pin; change internal lock