While 1947 pre-dates me a little, I think you will get numerous hits if you search these forums. Most will refer to a generally (I think former, as noted at http://www.eng-tips.com/viewthread.cfm?qid=154490 and other places!) practice of a man banging on the wall a vessel or pipe with a hammer, and even while under pressure.
 
I'm not sure about the "300 lb" part (unless the wielder is to be a Thor, or they are referring instead to a "drop hammer" test, that was I also think more infrequently done at various heights and with various much larger weights -- if so, the value obviously might not mean much in and of itself without more information/qualification).   

I believe the 300# hammer test was required by ASME Codes back in the early days as part of hydrostatic testing of welded vessels. The hammer testing was used in addition to hydrostatic pressure to ensure pressure retaining capability.  

..."300# hammer test...", that would be a big hammer.
When I ocassionally inspect a pressure vessel or a storage tank, I do a "hammer test" or more appropriately known as a "ring test" with a 10 ounce ball peen hammer.

Me thinks that if it was built in 1947 - it might be time to replace it or do a complete up to date testing of the tank.  That thing is like 65 years old!!!