The expected number of superimposed galaxies can be found by assuming all the sources to be in clusters and taking the mean cluster profile derived by Merrifield & Kent (1989). They derive a central surface density of ~400 galaxies/Mpc^2 falling exponentially with a 200 kpc scale length. Translated to the number of galaxies brighter than L*, this is a central surface density of 75 galaxies/Mpc^2. The sources in List A of GCL have largest linear sizes ~100 kpc so that less than one galaxy will be projected within that radius. The radio source covers only a small fraction of that area so that contamination by foreground galaxies shouldn't be a problem. The larger sources in List B will sample the smaller galaxy surface density at larger radii. The case of 3C275.1 fits the trend: it is approximately 100 kpc in size, covers a large fraction of the available area, is known to be in a rich cluster, and has a single superimposed galaxy with a depolarization patch.
Further examples would be useful, as this is a probe for the presence of magnetized gas in radio quiet galaxies. If the galaxies are in the cluster then one might expect the gas to have been stripped out by ram pressure. The galaxy in 3C275.1 has a much stronger depolarizing atmosphere than nearby radio loud ellipticals, as it is effective at a redshifted wavelength of 13 cm, which can be compared with the sources studied by Strom & Conway (1985) and Strom & Jägers (1988) which are only strongly depolarized at 49 cm.