Sorry for the poor quality — dark and stormy today + old phone pic
@pearsonified will have the answer. I think H-315 / HO-30RK is the fixed pipe model. I got a H-325 with sliding pipe.
You are correct—the H-315 is the fixed-pipe version of this pencil (flattened top of barrel with tapered flange leading to end cap), and the H-325 has a sliding sleeve.
Other than the tip/sleeve, these two pencils are identical.
(And for those who are too literal about everything, the H-325 is the same pencil as the HO-30RP. I prefer to use colloquial model numbers for Pilot because the catalog numbers are far too cryptic for public consumption.)