Its right there in front of you . 89 liters.
Or is it a 68L??
Nah it's a 89L Kombat
You can check it out in their magazine.
www.dropbox.com/sh/u2ykb9328l4rwb1/AAAcQxhhVMjhXnCE8H2dMZIVa/Starboard-Windsurfing-2008-Catalogue_compressed.pdf?dl=0
That is a custom made board, not a production board. It could have any volume. It looks very old 1985? It would be a terrible choice of board for you. At a guess, volume could be anywhere between 95-115 liters.

Hi
Whats the volume for this? can modern rig be used?
Assuming it is Trigger 9'6" to up to 10 feet say 115 to 130 litres approx (comparing it to the volume of a Bombora Tri Fin or the earlier 3 m Bombora Dromedary). Modern rigs may possibly work on it as it seems to have added a mast track aft of the original position, unless I have missed something. The original position is assumed by me to be in the black hole (forward of the mast track) from the original of around 1981- 1982. Try it and see as your photographs are not very clear given the camera angles. You can always change boom heights and/or run a higher mast base to fix the issues. Board will cost around $20-50 on its own so it is worth a try since it sounds like you are keen on a retroboard. There are plenty of 1980s sails around for next to nothing anyway. Good Luck.