Mark, this is a true SS396 Chevelle indicated by the VIN starting with 138. The trim tag decodes as:

02C: 3rd week of February 1966 is production build date, the 5906 is an order number I believe.

TR: 763-B (black bucket seat interior)
PNT: F F (solid paint in Marina Blue)

2 (options in this group are: L = 4 speed, G = console, and R = rear seat speaker)

The braces you see were on all factory SS Chevelles in 1966 and some L-79 4 speed Malibus that had the 12-bolt rear axle. I don't think that the trim tag has been changed from this car as it agrees with the vin as the car being built in Kansas City. I am not sure what the numbers on the block are but it does not seem that it is the original engine. You might have to look for the casting numbers and date and see how close it is for the production date of the car. I did not break down the other codes on the trim tag, only those that pertain to options and color........RatPack........