The missing Fiat "X Cars"?

I saw a Fiat X1/9 for sale on Trademe. Nearly did the heart over head thing and bid on it but I resisted.

I love the X1/9 but know that elderly Italian cars are not really a rational purchase. I’ve owned an X1/1 and X1/4 in the past but always wondered what the X1/5, X1/6 and X1/7 were?

• X1/1 became the production Fiat 128 (My second car)
• X1/2 became the production Autobianchi A112
• X1/3 became the production Fiat 130
• X1/4 became the production Fiat 127 (My first car)
• X1/5 ?
• X1/6 ?
• X1/7 ?
• X1/8 was at first stopped, but became later the X1/20 Lancia Montecarlo
• X1/9 became the production Fiat X1/9