The Supertech single valve spring described as 27.50 / 20mm CB 20.7mm chrome silicon is sold as a single unit and listed with those dimensional cues. Offered individually, this spring includes the stated coil and dimension references in the product description and is presented as the single-piece valve spring matching the described 20.7mm specification.
Supertech Single Valve Spring 27.50 / 20mm CB 20.7mm Chrome Silicon - Single