Do you think that might be a chain jack that was mentioned in one of the recent PRs?
It has hydraulics, and those 3-4" holes may be for it to slide back and forth on some rails as it jacks in a mooring chain.
here is the section-
The Messenger Wire Kit includes our standard fluid-compensated debris cover, which completely surrounds the BS Latcher Receiver. This unit is connected to a new, unique, high-strength, pre-tensioned messenger wire chain that is attached to a surface hang-off device, with an upper blind flange sealing off the I-tube and holding the chain under tension with a chain stop. The chain tension and extra slack chain is handled via a surface kit. This surface kit consists of a chain locker and a new double-acting chain jacking system which is mounted on the top of the I-tube hang off device. This enables the debris cover or subsea blind flange to be held under tension or lowered to the installation vessel in a continuous and controlled process. The Messenger Wire Kit also includes cylindrical drifts with internal anodes and port holes to provide circulation and optimum corrosion protection within the I-tube.
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