Not true. If the original buyer checked the box for SLP exhaust, you could have it on any year SS. My brother's 97 and 96 have every SLP option you could get.
When the SS first came out and you ordered the optional SLP exhaust you got the "two out the left" or TOTL. The exhaust code is 20TL. The systems installed in 96 did not have the cone of silence in them so they were pretty loud :yes:. This system also made the most horsepower because they flowed really well. Somebody (EPA) told GM/SLP that the sound level was too high and to fix it. The solution was the infamous "cone of silence" which is essentially a tapered washer welded into the end of the exhaust pipe where is goes into the muffler. It reduces the diameter of the pipe from 3" down to about 1.5". When my brother took his out - big difference in sound - and seat of the pants too. Anyway, sometime in mid year 1997, GM/SLP decided to get rid of the TOTL because of the noise issue and people bitching that it didn't have a dual exhaust. They started installing the dual/dual outlet pipe instead of the TOTL sometime in the midyear. This system didn't quite flow as much but was quieter and also looks pretty good.
So: An original 1997 SS could have one of three exhaust systems and be 100% correct. The SLP TOTL, the SLP dual/dual, or the GM Z28 set up which has two chrome RECTANGULAR exhaust outlets.
BTW, of the 957 1997 30th Anniversary SS cars, 871 had the optional SLP exhaust. If you have one of the 30th SS cars with the stock Z28 exhaust, think real hard before you change it! There were only 38 30th Anniversary SS T-Tops, 26 coupes, and 22 convertibles sold with the original GM Z28 exhaust. I'd imagine the numbers are very similar for the regular 97 SS cars.
Hope this helps!