1968 International Scout Battery

I purchased the Scout in June. It had been looked at by my mechanic and he gave it the ok. The scout has been redone and approx 1800 miles on the rebuit engine. It is running very good. I had not driven it in about a month. I drove it for 2 hours on Saturday. Stopped at the grocery store and the battery was dead. This battery is fairly new (1 yr). When I jumped it, it started right up. No problem with cables but, seems to be losing battery. It started yesterday afternoon. Let it run for a few minutes. Later last night was dead again.

Any ideas?