Page Publications' Gunsmoke Western Picture Library #4 has a $0.20 cover price with a Keith Chatto cover.
A Trap for a Killer! is the first story in the issue, featuring Kid Colt, Outlaw.
The advertiser's date code on the rear cover is PP 1.72, which suggests this issue was on the stands very late-1971/early-1972.
The issue was reprinted shortly afterwards by Page Publications as Gunshot Western Library #27:
This issue has a $0.25 cover price and a new Keith Chatto cover.
The "5" marked in texta may be a newsagent's marking referring to the month of May. I picked this issue up in a batch which included Western Adventure Library #7, which also has "5" marked in texta:
So whilst it's inconclusive, for the moment I'm going to run with the idea that both these issues were on the stands circa mid-1972, which approximates the guestimates on AusReprints.
Western Adventure Library #7 also features a Keith Chatto cover.