The O2 Cylinder that fits the ASW 27 is a Norris Cylinder, Huntsville AL, Drawn Medical D Cylinder, Drawing number: 3740416-00
The Cylinder OD on the drawing is 4.150"
The cylinder that I have, that fits in the aircraft has an OD that measures 4.196"
This is a drawn piece, not machined, so the OD isn't something that is going to be a closely held dimension. My cylinder is painted. The 4.150" is probably an as drawn dimension. If you order one from them, I would specify a Max OD of 4.2" They may have to pick through their inventory to find one that isn't over this. I have a cylinder drawing for anyone that wants one get in touch with me at this email address without the X's: 211 XX fletcher xATx gmail DOT com

SF