Yes, no tag is normal for San Jose builds... Most San Jose cars did not get delivered with buck tags. The tags were removed prior to completion/shipping...
Just a little correction. San Jose in 68 didn't use or make buck tags so they were never installed nor removed. Each plant handled some production practices different possibly due to what had been done by the workers in prior years, the way the plant was laid out or the manager just wanted to do it his way and it worked. Sort of like the different way that Dearborn used their buildsheets when compared to San Jose practices
San Jose first started using buck tags in 69 (trying it out) and used them on all 70 Mustangs before production of Mustangs was shut down the end of that production year.
As for a site to decode them more will be difficult to use since each plant used different codes during different time period according to the guys that made up those codes. If the site doesn't reference codes by year/month and plant its going to get confusing and possibly provide you with the incorrect answers to your questions. Best IMHO and most recent publication of the codes can be found in The Mustang and Cougar Tagbook by Kevin Marti