Originally Posted by fernando
A Portuguese cavalry crossbow, used by King Dom Sebastião (1557-1578) personal mounted guard. The brass initial S can be seen on the stock.
Thank you so much, 'Nando,
This sort of early Renaissance crossbows first of all appeared in Northern Italy, with one of the earliest being made for Maximilian I in about 1508, and the style was soon adopted in Spain in the 1530's and later in Portugal.
The one you illustrated can be safely attributed to ca. 1570. There are simillar but earlier samples preserved in the Real Armería Madrid, coming from Charles V's armory. The oldest features of these are the long 'Gothic' trigger which of course by then was just a trigger guard.