Thanks Tom. I just measured a broken cylinder on a pump for sale and I measured the same size as yours. The one thing I noticed is the 12-1/2' diameter glass butts up real hard against the bottom lip with no room to seal the cylinder for leaks. A 12" diameter cylinder would allow you to seal it at the base. My G&B 176 and Wayne 515 has the 1/4" gap all round the base for sealing. The acrylic cylinder sellers specify a 12" diameter but as you say, the original size is 12.5" ? I must say I am a little confused.