Manufactured to military specification, these are a large sized heavy duty anti shock anti vibration mount.

Made by Cementation (Muffelite) Ltd as their part number GBC-2125-T6, these are also known as Barry mounts and Muffelite mounts.

The square base measures 75mm square, with mountings at 64mm centers (approx). The top mount is about 75mm in diameter with what looks like a 6mm threaded mounting hole in the center.