If you need a cylinder replaced, let a qualified gun smith do it. If it needs to be timed, you won't regret spending the money.
The Colt Trooper MK III .357 with serial number J11918 was manufactured in 1975. Colt's production of the Trooper MK III series began in 1969 and continued until 1983, with serial numbers indicating the year of production.
about 1973
Look on the butt.
your colt trooper mkIII was made in 1978.
Colt data at Proofhouse.com
100-500 USD
1958
No serial number provided.
Around 1973.
50-650 usd
You must call Colt to find out.
Between 1971 & 1972