Aside from the front with different meters and a sensitivity switch, nothing. Or at least I think so hmmm.... working on David's 700B (first one), it did seem that the xformer leads were a little more substantial on the series I. Probably a slightly different design, but Joe n Lee would be able to say for sure. In any case, from the transformer to the output terms is all the same "schtuff" on what we do with 'em.
But yeah it's those meters and pretty lit center display that make me want a "B", eventually... heheh