I have a small steel round bottom tank. It's 16 3/4 from the bottom of the tank to the top of the neck. It's 6,9 diameter and 2250 PSI. SCUBAPRO marketed a 40 circa 1970 and I believe this may be one of them. If anyone has one they'd be willing to sell or trade for other dive gear, preferably near the Mass-NH border please let me know