Reisetomate is probably a fasciated mutant

I had to look hard to find a description of the Reisetomate phenotype in the literature.  However, an allele of the fasciated gene seems to generate this phenotype.  fasciated (fas) is a well-documented gene that increases fruit size and changes the fruit's internal structure, by increasing locule number.  In fact all "beefsteak" tomatoes probably have mutations in either this gene, or a similar-functioning gene called locule number, or both.   The fasciated mutation in these varieties seems to increase carpel number.   So does Reisetomate - but with a key difference.   A review article by Steven D. Tanksley has a photo of a clearly Reisetomate-looking funky tomato, and he describes it thusly:

"...Moreover, one mutant allele of fasciated is associated not only with more carpels but also with unfused carpels (Figure 1D)".   
The Plant Cell June 2004 vol. 16 no. suppl 1 S181-S189

Because the carpels remain unfused, the result is a fruit with separate locules that are not really attached to each other.  Neat.

Here is a comparison of Reisetomate vs. a "ribbed" tomato, Costoluto Genovese.   The latter probably has a different allele at fas or a similar gene.  I think.