The weight is what counts. Your usual 232AH battery is 67 lbs so those 63 lb batteries are the smaller AH versions, like US Battery makes U-2200s (67 lbs 232AH) and also makes U-2000 (63 lb 200AH ISTR) and they also even make/made a U-1800s at 190AH whatever light weight)
This is why you need your bathroom scales with you when buying at Sam's or Costco so you can ignore all the funny labels and just weigh the thing to know which version in AH of the "real brand" you have there. IMO don't buy any 6v batt under 65lbs.