Keith,
Can't help you where it was mentioned about shell for 6 pounder field guns and the 12 pounder field howitzer. Going out on a limb, I feel shell was little used for these types of field cannon because the danger of the short range of weapons and the need to cut the fuse short because of the short range. They certainly could have been fired over the heads of the infantry but they would not like that. I read online that shell was available for the 12 pounder but web site made no mention of 6 pounder shell.
Regards Ron