The single pass exchanger was an absolutely gorgeous fabrication of unknown origin. I found it on eBay for $75 and after learning it was unsatisfactory, sold it on eBay in 4 hours for $75. The gentleman who bought it is using it in a chemical process, I suspect that was the intent of the design in the first place.
As I said in a previous post, neither the Moyer nor Indigo systems would work for me due to zero clearance at the flywheel and a water heater in the loop mounted higher than the engine. Because of that I was left to my own devices to assemble a system. For those in a similar position, an important feature in selecting an exchanger besides the multiple pass design is the internal tube bundle. It's common in engine type exchangers, not so in others. My original exchanger didn't have the tube bundle and that should have been a red flag right there. I treated it as a yellow flag and although it didn't cost me any additional money it did add about three weeks to the project finding and installing a replacement.
Here's a pic of the unsatisfactory exchanger. There were no markings on it anywhere but it had the look of professional manufacture, certainly not home grown.
FWC 001.jpg
The dimensions were similar to other exchangers used successfully on A-4's. 3" diameter x approx. 16" length. I learned the hard way that what goes on inside means more than dimension.