I removed the original Wilcox Crittenden head from my 1977 vintage M-17 last fall and plan to replace it with a Porta Potti. Where I sail there are no pump-out facilities. I'm considering either the Thetford 135 or 735. So far I haven't found a dealer nearby who has either in stock that I can look at. As near as I can tell from the Thetford web site, the difference seems to be the flushing mechanism - piston pump on the 735 versus bellows on the 135, whatever that means. Does anyone out there have wisdom to share on either one? Also, my old but fully-functioning Wilcox-Crittenden head is looking for a new home. I rebuilt it in 2004 and it has been rarely used. I'm located in the Twin Cities (MN), and I'm open to reasonable offers. Rich Makela M-17 #233 - Harmony