[Dave at Pelican Parts]

They are the original 914-6 brake calipers, as well as being the ones used on the 911 from about 1969 through 77 or thereabouts. Their mounting bolts are only 3" (on-center) apart, and they are made of steel and so a magnet will stick to them. You need 911 M struts or 914-6 struts to use these.

The "S" calipers are aluminum (lighter, better heat dissipation, more prone to flex, magnet will not stick) and have a 3.5" mounting bolt spacing. They were an option on 911s from about 67 up through 79 or 80.

The "A" calipers are steel and have a 3.5" bolt spacing. They were the non-S option on 911s from the end of the "M" calipers up until 89.

--DD