It's believed that serial number start at 8MA500 and only even numbers were used. So that would make yours no 253. Other factors are also to be considered yours is a 4 headlight car as in early cars, later US cars had 2 headlights.
a good source is http://www.mangustainternational.com
Denis
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.