6v lantern battery amp hours?

The amp hour rating of a carbon zinc cell depended on the usage. It would last only shortly with heavy usage because the manganese compounds could not adsorb the gas generated around the carbon electrode. Long tern usage at moderate drains would be about the same as a carbon zinc D cell. The guts of the heavy duty lantern battery used 4 cells that were about the same as a D cell but had a thicker zinc outer container. The carbon electrode seemed to be a little larger but more porous and delicate. The combination would give higher peak powers and a higher overall amp hour rating when used with moderate drainage. The over all limits in amp hour rating was shelf life. If you got one that was 2 years old. lifetime could be measured in hours under any load. Back then, at the height of the carbon/zinc cell, you always asked the hardware store owner when he got his shipment. You could always get a few extra hours out of any of them by baking at low temperatures in the oven. This caused the ges around the electrode to dissapate and the internal resistance to go down. Not the answer you looked for but if you can find something for a d cell you can estimate. The other trick is to find someone with a chart recorder. Put the voltage on one axis and the time will be the other. A moderate load is placed across the battery. Hopefully the amperage will be the same at the start and the end. Time how long to get to half volltage. Take the time and multiply it by the current and get about the amp/hour rating.