I would suggest getting a barcode scanner (assuming you don't have one or that you can afford one). Barcodes started in the mid-70's. However, at the beginning, they used the same numbers over every year (i.e. the number used for Feb 1978 would be the same number used for Feb 1979). By the 80's, I believe, they were using the longer, more unique codes.
CB has a lot of these already in the system (but not all, by any means).
By using a barcode scanner, assuming the number is already in CB, with the 'Add by Barcode' menu item, it will find the issue being scanned, even if it is a non-intuitive location.
If you don't have/want a barcode scanner, you will need to find each issue and change the Qty (in Stock) from 0 to 1 (or more if you have duplicates).
If you know the Grade of the issue, you can change it on the 'Add by Barcode' window.
Or you after entering the issues, you could use the 'Grading Item' menu option to get a general idea of the grade.
If you have the Pro or Archive Edition, you can set up Custom Fields for thing like Location (box number).
If you can't find a particular Issue, post information (and a cover scan would be helpful) and someone may be able to help you find it.