I need a replacement gear housing for my stihl HS81R. (Serial number 172069212.) I see you list two, what is the difference between the two?

There are a couple of variations of the HS 81 R, Which is why there is a couple of options on the website, the gear housing for your machine is on part number. 4237 640 0501 https://www.lsengineers.co.uk/catalog/product/view/id/25173