I'm not an expert on Zexel pumps, but I'm near certain you're describing a delivery valve (barrel) that is height adjusted for proper flow timing. This is done at an injection shop by spotting the pump timing for each delivery valve and adjusting each valve by screwing delivery valves in or out to achieve proper timing. Once proper flow timing is achieved, the clamps that lock two delivery valves are fitted to prevent timing from being upset.
About the best you can do in the field is remove the injection line, then take a dial or digital caliper and note the exact height of the delivery valve before you remove the retaining clamp. Upon re-installation, turn delivery valve in until that proper dimension is achieved.
Remove delivery valve with care, you may find a seat, pintle, and spring inside there.