I have a couple of books to decode the VIN on my 69 convertable. The axle code on my data plate is "13" but according to both books, there is no such thing as a "13" for an axle code.  Any ideas?  

There have been numerous cases over the years of miss-stamped data plates.  What engine and transmission do you have (or on the data tag)?  Does the car have A/C?   A "3" code was a 2.79 in 1969.  If you have a 302-2V non air car it's probably a 2.79 ratio.  Air conditioned cars usually came with 3.00 ratios.

Have you ordered a Marti Report?  

... another double post, sorry.    

It would be interesting to investigate this further.  I guess I need to get around to ordering that Marti Report. The car is a 302-2V without air. Thanks for the info.  

It's probably a "miss-stamp". The normal axle ratio for 302-2V without air was a 2.79 which should be a "3".  A Marti Report would confirm it.