Mostly they were two-rope grabs:
- one rope attached to the top block that anchors the 4 x arms to bucket outer corners,
- one attached to the lower transom to which the bucket shell inner pivots mount.
When the load is taken by the top block rope, the bucket shells open by virtue of its weight.
When the lower rope takes the weight, the transom is lifted up relative to the top block which closes the bucket shells.
You can get single rope grabs where there is a latching lock that can be released by a tag line.
I'll take a look at the Sentinel pics again to see what type these were.