I think the "A" and "B" series make sense, since from the different is odes that talk about Soong I always got the feeling that Soong had made some androids that had failed and he was "laughed at" by his other colleagues into hiding, because his earlier prototypes failed completely, after building up a big expectation for it. So B-4, Lore and Data could all be part of his redesigned series. Plus I believe that in a few episodes it was mentioned that Data had an R type positron if brain (I think it was mentioned in "Time's Arrow"), so who knows if Soong created a body for each type of brain. There could've been brains made but no bodies.
But with the large leap from B-4 to Lore, look at the large leap from Data to Juliana (and I don't just mean Soong created a female android), but after Data Soong added the ability for Juliana to scan as human, and operate as a full human.