So, I have a 78 L48.  The question I have is about the trim tag.  I understand what most of the numbers and letters mean.  However, when I was trying to decode the numbers I found something strange (at least it seems strange to me).  My trim code is 68C.  When I looked this up I could not find any a match for 68.  My interior is a dark brown, so I would think that the trim code should be 69C.  But the tag is obviously stamped with a 68C.  Anyone have any ideas?  Also, my paint code is 89L.  I know what the 89 stands for, but what does the "L" mean? 

The L in the exterior paint code denotes Lower. They started using the "L" in the paint codes around 1973, since they were starting to do other car lines in two-tones("upper/lower"). If the car wasn't two-toned, it only used the "L" in the code.

I can't find any mention of an interior code of 68, period, in any year Shark car. Your's is either a mis-stamp, or someone has messed with the tag...I'd be most inclined to believe it was simply a mis-stamp, but...it could also have been a special order color. As long as that particular color was available anywhere(on any car) in the GM line, it could be used, if ordered by the right people.
The C in the code is for cloth interior.
Can you get a good pic of the tag, and post it? I'd kinda like to see that, too...it is a little strange, but not totally unheard of.

I do have a picture of the tag.  I'll see if I can get that uploaded here today. 

Sorry, I meant 68 TRIM code.  not paint code. 

Yeah...that's definitely a 68C...wow. The 8 looks just like the 8 in the paint code, so it doesn't look like someone has tampered with it at all.
My guess would be it was a special order deal.
Does the interior appear to be the original for the car? I'm wondering if it was a special order car, and then somewhere down the line in life, someone changed the interior ,and just put in the "correct" dark brown?  I wonder if the carpet still has the original tag on it, that would give a clue. The tag on my rear carpeting was still on mine when I installed the new stuff years ago, and it had the code, and date on it...

"Exterior and Interior trim combinations listed are those recommended and available; other combinations and special exterior colors/primer other than those listed were possible."

It appears to be the original interior, but I'm really not 100%.  I'll have to check the tag on the carpet.  I guess I didn't even think about that.  The interior looks like a dark brown to me, but maybe it's slightly different or something.