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the spacer is a means of fine tuning the cycle's chainline

they are typically offered in three thicknesses: 1.0mm, 1.5mm & 2.0mm

many/most shops and workshops stock only the 1.5mm thickness so it is "that or nothing". if you measure what you have there you shall likely find it to be the 1.5 size.

the Regina Extra is the manufacturer's economy (base) model gear block

yes, you could run a 6V gear block if you wished; it would need to be a narrow six such as the Suntour Ultra 6

six cogs in the lateral space of a conventional five


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