This comes up about every 2 months where a defunct company's card is questioned about why doesn't mine have a serial number.
There are 3 reasons why you can have a copy of card that was supposed to serial numbered, but yours is missing it.
1 - Serial number stamper missed your card and it got out into packs. (Rare, but happens)
2 - Extras were made (all companies made extras) and one that was in packs was somehow damaged before the customer opened the pack, and they returned it for a mint copy. The mint copy was returned minus the serial number (in some cases, an employyee may hand write the original SN).
3 - When the card company went bankrupt, a liquidator came in and sold off all the supplies and inventory in their warehouse(s), including the extras that were set aside to be replacements. These find their way to ebay and the like, and are sold without the SNs, actually also meaning that there are now an undetermined "extra" amount of these cards in the hobby.
In any of these cases, they are not listed here as a variation or anything like that.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 10/25/2023). I have added a large portion of my inventory to the site, and currently have trading turned off (details are in my profile). I have (finally) unearthed my 2 very large boxes of Star Company minor league sets and they are available (email for details).