this is the transmission rubber used with the crossmember.it's off a stanza (a10) or an a14/15 sunny.probably off lots of other datsuns,too.the studs out the side are for an exhaust system mount.
this is the u-shaped plate by itself.you should be able to see the welded on spacer on the arms.the holes have been slotted because it was usd in 3 different cars & they all seemed to have different positions for the mounts.big washers were always used to hide this.