I was looking at both Kirban models on-line... since the base unbolts from the top to replace the diaphragm, it looks like, if you can just get the 2 bolt base of the 5018 model you could just bolt it to the upper body... I sent a e-mail to the company to confirm that and if there was a way to buy/order that bottom/base part from them...
I noticed that too. Let me know what they say because if they do allow you to buy just the bottom part, I would get one as well