Hi Kate,
I would not worry about it my boat has exactly the same, i spoke to Bill about it and he had no concerns either.
The cause is a bit of moisture finding its way under the slab and rusting, the cracks are due to the expansion and contraction of the slab in different temps.
The slab is lowered (this explanation is for a P27 but i assume the 275 is similar) in the the polyester resin which is not part of the hull lay up.
My slab actually sticks out abit proud at the aft end when dried out on its cradle but the slab is still held firm.
Some day when i get round to it i intend to scrape out the rusty bits and the cracks and tidy it up with resin.
But just incase i have misread your interpretation it will be worth a call to Bill.
Sean