The Closed Circuit appears in the 1949 Claud Butler catalogue in '531' with 72½° parallel angles and a 42¼in wheelbase. Three versions were listed: Model 7 (Mk I, welded, Simplex 8-speed), Model 6 (Mk II, brazed, Simplex 4-speed) and Model 8 (Mk III, brazed, club build).
Known components
Listed exactly as our confirmed sources give them, with the year and source for each. A part listed for one year or version may not apply to every bike, so check against yours.
Frame and fork
- '531'1949 · 7 (Mk I), 6 (Mk II), 8 (Mk III) — Ebykr catalogue archive, p.5
As listed together
Parts the source lists in one breath, kept together so nothing is attached to the wrong version.
- Mk I welded with Simplex 8-speed, Mk II brazed with Simplex 4-speed, Mk III brazed club build1949 · 7 (Mk I), 6 (Mk II), 8 (Mk III) — Ebykr catalogue archive, p.5
Catalogue appearances
| Year | Name as printed · model no. | Details | Source |
|---|---|---|---|
| 1949 | Closed Circuit · 7 (Mk I), 6 (Mk II), 8 (Mk III) | '531'; 72½° parallel; 42¼in wheelbase; Mk I welded with Simplex 8-speed, Mk II brazed with Simplex 4-speed, Mk III brazed club build. | Ebykr catalogue archive, p.5 |
Specs are as printed in the catalogue. Years are when the model appears in a source, not exact production dates.
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Sources
Last updated 19 October 2026. Every fact on this page comes from one of these.