This is a "benchmark". It's installed by the US Army Corps of Engineers and used by the United States Geological Survey (USGS) to mark at site that coincides with marks on maps they create. In the early 1900's the markers included a lot of information right on site. Now there is a reference number. In this case I assume the BC on the monument stands for Branciforte Creek which is about 100 ft away. GPS has made it possible to greatly increase the accuracy of these BMs. For instance, on the top of Mt. Whitney, the highest point in the contiguous US there are at least 8 of these BMs. Each on dated and more accurately displaying the height of the peak.
Not far from where I live stands an old multi-car garage designed to accommodate summer visitors that stayed in these cabins. It was the 1940's and Santa Cruz, CA barely had 6,000 people living here. Now the population is around 60,000. Right behind the cabins was creek. It used to run year-round and was a great semi-shaded place to enjoy the warm summer and clean clear water. There are several "Church Camps" in the area. Most are still operating under church supervision but this one is privately owned. The creek is an aqueduct now. So much development upstream has increased rapid runoff so the stream had to be converted for flood control. It's a charming little secret spot that is the source of many childhood and family memories.