what exactly are these?

[Kapt Z]

Prop guards. Always thought they were there to keep the props clear of piers and other obstructions in port. Never heard about their use against dock lines, but I guess that makes sense as well.

It was my understanding that they were temporary 'in port' attachments and were removed prior to leaving on patrol. If that is true then it is strange that Grunion would have had them when she was lost.