The rondell doesn't necessarily mean pre-Cadogon. I've encountered pipes with the round COM and a rondell, and refer to them as early Cadogan era pipes.
I believe you describe your pipe's COM as below, which is post 1981. This example, a shape 5 did have the brass rondell.
Also muddying the water, GBD's produced in the late 1990's to early 2000's also used a brass rondell and went back to the straight line "London, England" COM. These pipes often had brass accents, like the aforementioned Garland.