Per the SCSW, the first year of production for the model 60 was 1965, the R serial number series for the model 60 started in 1969, and s/n R1152xx shipped in 1974-75. Also per the SCSW, the model 60 shared an all-numeric s/n series with the Chief Special until 1969, when the R s/n's started.
FWIW I recently bought two model 60's, both in pretty nice condition. A couple months ago, I paid $425 (private party sale) for the first one, a 1979-80 gun, and just last friday, I paid the same $425 out-the-door ($390 + tax) from a LGS for a 1970-73 gun. I feel like I got a fair deal on the private party buy, and a good deal on the gunshop buy- esp since the gunshop gun included the original grips s/n'd to the gun.
So if the OP is gonna sell great-unc's model 60, $400-ish is a fair price depending on condition. But I'd recommend he keep it-- like all Chief Specials, the model 60 is a great little carry gun and they're just not making them anymore.