SML 

Metal Machine Spools - Loose

66K, or L-style bobbins, are slightly narrower (~9mm) and are necessary for some machines, but can also be used in 15K (A-style bobbin) machines with a washer.  15K (A-style) bobbins are slightly wider (~10mm) but are the most commonly used bobbin style in the UK.  Both styles can be used in domestic machines as well as some commercial embroidery machines.  The know which bobbin you require please refer to your sewing machine manual.