Hello and welcome.

It's not a signature on the base, just the factory mark with a date.
The script looks mostly right, but I'm not used to seeing a loopy Mdina one like that, executed with a thin line, from a period as late as '87. They'd moved to using a vibrating tool by then and the mark was a lot thicker.
The date figures look
sort of right too, but I'm not sure the marks haven't been added at a later date by somebody else. I can't say for sure thay they were though.
Not that it really matters.

You do have an exceptionally good and big Mdina Fish, the date on it is consistent with the design.
We don't discuss values here really, they're far too contentious. Something is worth what somebody is willing to pay for it, in that place, at that time. But you can check previous sales in online auctions.