It is stub on the regular bolt (ie a double end bolt) for mounting stabilizer bar on many garden tillers and other equipment that create a large torque changes during operation.
Thanks AVB I have a feeling this was not the original motor as it's a 92 model and I don't think Rover were still making the Thoroughbred cylinder mower at that point. May have come off something else. Cheers
AVB is correct that they are usually used as a tie down strap
However, Alroh cylinder mowers used to tension the engine to cylinder belt by tilting the engine by means of a lever on the handle bar, attached to a steel rod that bolted to the front left head bolt
This one is not a good example, but this gives you the general idea
