T-D,
Looking at your pictures, it shows the serial number for a .45 Colt Blackhawk from 1972. You can check this on Ruger's website,
http://www.ruger.com/service/productHistory/RE-Bhawk45.html
Ruger first made a 45 Colt chambered Blackhawk in 1971.
I think the term "contoured gate" would apply to the Single Six model, in .22 caliber, which originally had a "flat gate" then later, in 1957 or so, changed to a rounded loading gate.
Factory engraved Single Sixes did exist - about 260 of them in the same time frame - 1956 to 1957 - but Ruger had still not manufactured any .45 Blackhawks at that time. These factory engraved Single Sixes are quite valuable, similar to the dollar amounts your gun dealer is quoting.
There are lots of folks on this website that can help identify your gun. You could also try the Ruger Collectors Association for help.
Also, give Ruger Customer Service a call and they will tell you the month and year your gun was shipped to clarify further.
Good luck - nice looking gun!
Tim