That span is a rare variation of a retractile type movable span. I call it a variation because the span must be raised a short distance before being rolled back which is unlike most retractile bridges.
Not mentioned is that fact the SM Long bridge has a lower deck for the railway (now just a spur to the Naval Shipyard) and that lower deck has a drawbridge, usually left open, under the fixed span on the Maine side under the fixed span.
The attached photo is by JayDuck and copied from the Wikipedia page for the Long bridge.
The National Bridge Inventory lists the "Navigation Vertical Clearance" as 68.9 Feet (41 meters). This may be when the bridge is raised.
You have done a marvelous job of describing a part of our seacoast culture. Your condition report shows you have a good grasp of important detail that takes this report into the seriously useful data and sets it apart from idle artistic information.
However, there is one critical piece of data not reported. What is the vertical clearance above mean seal level at maximum lift?