The Rapid Rectilinear is symmetrical (has two identical achromats), so is convertible. If you take the front one off, you have the preferred stop-in-front-of meniscus-achromat configuration, but twice the f.l. and new f-#.
According to some references 'anastigmat' types can be 3 or more element, well-corrected. Some evolved into or were similar to Tessar's, and thus can't play without their proper combined halves, which are not at all symmetrical...and don't work standalone (the ones I've tried).