This is the same Oberdorfer pump that is used on an Atomic 4. The folks at Moyer Marine Inc (MMI) know all about this pump Moyer Marine Atomic 4 Engine Rebuilding and Parts
and the rebuild kits and are actually really great on the phone, IMHO.
Another place to look is on their forum using the search function for 'Oberdorfer' or 'water pump': Moyer Marine Atomic 4 Community - Powered by vBulletin
Their prices for parts etc. may not be as competitive as E-discountmarine.com but they will speak to you on the phone and good customer service is a rarity these days, especially from someone who actually knows what they are talking about.
Full Disclosure: I am a satisfied customer of MMI only and do not in any way work for the company. My Atomic 4 is 42 years old and still kicking over mostly because of MMI and my PO's and my own maintenance.