It looks similar to this Selden one.
Gear: Selden’s HTI Hydraulic Backstay Tensioner
I cannot see in your photo if there is a manufacturer’s name on the pressure dial.
I have a Navtec hydraulic one on my American built J-boat and get it serviced by Allspars, Plymouth, who may be able to help...
Yes, sad times indeed. However, a friend on the Dartmouth Sailing Week committee says that they are still hoping to put on an event depending on what the lockdown position is at the time. We shall have to keep our fingers crossed.