curiously, the 'author' of this sentence seems to have lifted it wholesale from this book:
https://books.google.com/books?id=dz6bO ... 22&f=falsein any case,
by itself this is a bad sentence—it doesn't make sense without the larger surrounding context provided by the book.
if this sentence appears alone, there is no reason to write the first verb in any tense other than the simple past ("...was not signed until...").
the tense used here makes sense, but only because it appears (in the original book) in a context that suggests a sense of seemingly interminable delay.
also, the use of this tense is very florid and 'literary'. it's not bad writing
per se, but it clashes sharply with the drier, more matter-of-fact style used by GMAC.