Hello Bernardo.

As Gianvittorio says, without much to go on we can only give you some basic information.

The L5100 movement itself dates initially from 1978, although Heuer didn't pick it up immediately. Heuer passes from family ownership to a group including Lemania in 1982 and appears to start using the L5100 the next year, so our baseline date is 1983. Ownership changed again, this time to TAG, in 1985 after which point only a few Heuers continued with the movement.

So we're talking quite a narrow window for most of the watches, 1983-85, extending slightly for 501.500 through 510.504 but not for more than a year or two. Hope that gives you some idea at least.

Worth reading DC's pair of articles on the relationship between Heuer and Lemania:

http://www.calibre11.com/heuer-lemania-part-one/
http://www.calibre11.com/heuer-lemania-part-two/