I offered this suggestion to someone privately the other day, and it could work well for older (non-serial numbered cards, such as your 1981 Hendersons), use the Serial Number field to show how many of a particular card you have. This shows in the collection summary (unlike the Comment field, albeit with a # in front).
Just an idea.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).