Given the context and wording, I would say the iron weight is symbolizing a scale, or a like instrument used for weighing things such as potatoes or hay.
The phrase an "iron weight" most likely describes a device that is used for weighing potatoes and hay. We know this because the second part of the sentence explains what the use of the machine is. Moreover, we know that the object is made of iron, which means that it is a concrete object. If the character was threatening someone with it, it is likely that he was threatening him with throwing the device at him.
