This is a Polatronic flash bracket (or mount), No. 8402, which was used with all SX-70 cameras including non-folding plastic models. It was meant to be used with the Polatronic flash, model #2350. The bracket could also be used as a tripod mount. It was made in Japan.
This bracket was sold with or separately from the Polatronic flash.
This accessory is not described in the very useful Land List
website, but the
Polatronic flash #2350 is.