In "The Big Bang Theory" episode "The Closet Reconfiguration" (S06E19), the gang gets Sheldon to tell them the contents of a letter he found in Howard's closet from his father. Later, they give Howard various theories on what it contained and he declines from knowing which one was actually true. Of all the options, Bernadette describes a picture of his father holding Howard on the day he was born and on the back was written: Howard my son, my greatest gift.
In an episode in the next season: "The Thanksgiving Decoupling" (S07E09), in a scene set in Howard's house, there is a picture of a man holding a baby in the background (in the stairwell; at around 04:52 into the episode). Could this be the contents of the letter? Making Bernadette's theory the correct one? This would include the assumption that someone removed the contents from the envelope as Howard lights it up and then throws it into the sink. Or that they managed to salvage it before it was completely destroyed.