Tim & Denny,
Am I confused (as usual)?
I think Tim's style of fitting (1/8" compression nut (Eaton 6100x2 1/8" Threaded Sleeve Nut - no need for a ferrule with this) is used at the engine-end of the copper tubing. That fitting is also available by the piece at NAPA (and probably elsewhere - Eaton makes it). If you want a NAPA number, let me know.
A 1/8" cap (Nut) and ferrule are used (I think) at the gauge end. And, thanks to Denny's info (if I understand his post correctly), it looks like that arrangement was most likely used from 47-55).
I'll go check my 54 gauge assemblies to refresh my failing memory.
