I realize the FullRiver FR-31 is not on the FullRiver website and suspect that FullRiver may make it for certain distributors like DC Battery or possibly it is re-numbered for DC Battery.
The FullRiver FR-31 that is in a brochure at DC Battery is the battery that we purchased. More that this, I cannot say.
The photo I uploaded to the photo section of the site titled Battery Compartment shows a total of 13 batteries in BeBe, #387 and I am fairly sure that the compartment is the same size as yours.
s/v BeBe, SM2, #387