According to the Smiths list the RSM 3003/00 would be the correct magnetic tacho for an Atlas* (from '64?). The "A" likely indicates a later replacement production run or minor alteration so would still be correct but probably not the original tacho.
*(Also 650, P11, N15, various Matchless, etc.)
Your RSM 3003/00A tacho, therefore, should be marked "RPM, x100,
2-1"...
... the matching drive gearbox would be a
1:1 BG 1508/05.
Condition: NOS Made in England by Smiths Industries Used for checking ratio on bike for one minute only.1-1 RatioWill fit BSA Gold Star (65-9164)
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