Well it does look like there should be a shaft going aft from the cylinder but it is hard to tell. Perhaps there was a problem with the mechanism and a "thrifty" DPO (dreaded previous owner, who gets blamed for everything that is wrong with the boat, even if you did it!) decided to disable it. I doubt they would disable a functioning one. But perhaps they sealed it off with a plumbing plug as that looks to be what is there now. I would start off with removing that to see what is behind it. I have heard of people sealing off a center board when they sail predominately in shallow waters, and light airs. But mostly I think it normally just a kludge rather than fixing the real issue. I have not seen someone do it to a boat as small as this. There have even been builders that just deleted the swing keel, and called it a shallow draft keel. What to do would depend on what was broken. At this point I would sail it as is for this season and at next haul out you might want to do some exploratory grinding. If they did a clean job of covering the opening it might not be hard, but that is not likely.
I think it depends on your attachment to this boat, you might be better off starting fresh. After another year of sailing you might want more boat anyway. Those stringers don't look too solid but it is hard to tell from the photos. So perhaps it might be worth a general condition survey to see what a professional would advise. Really depends on your patience for projects, skills, work-space and tools.
Look at number 4 on "how to unstick my centerboard"
FREQUENTLY ASKED QUESTIONS ABOUT GRAMPIAN 26
I think you will find what they did to fix it. I guess it depends on how much you have invested. I would likely sail it as is, till I moved up.