score:13
What you are looking for is the Battle of the Falaise Pocket, which took place from 12-21 August 1944. Chambois seems to be the spot where the pocket was actually closed, or where the encircling Allied forces met up, which took place on 19 August.
According to Wikipedia,
the battle is also referred to as the Battle of the Falaise Gap, after the corridor which the Germans sought to maintain to allow their escape, and is sometimes referred to as the Chambois Pocket, the Falaise-Chambois Pocket, the Argentan–Falaise Pocket or the Trun–Chambois Gap.
The German Wikipedia calls this the "Kessel von Falaise".
Wikipedia further explains that disagreements between Patton and Montgomery, leaders of the American and British forces, allowed much of “the beaten but still coherently organized German army” to escape.
A “Selbstfahrlafette” is basically a tank (or some other type of vehicle with mounted gun). So it seems your grandfather was in a tank trying to break through the encirclement, and in the process he was separated from his unit.
Upvote:4
A 2 cm Selbstfahrlafette seems most likely to be a Halftrack with a 20mm anti-aircraft gun - probably an SdKfz 7/1 or Sdkfz 10/4 though it could also have been a wheeled truck. (Fully tracked flakpanzer tanks were quite rare).
Either way it would be a very unsuitable vehicle for any kind of ground engagement in the circumstances of the Falaise Pocket and extremely vulnerable to return fire. It's likely that they encountered Allied forces, abandoned the vehicle and were then captured.