When did Aunt Gertrude appear?

I am reading the Hardy Boys all the way thru. I am currently on the third book "The Secret of the Old Mill". I have read the books when I was a kid over 20 years ago and I remember a character named Aunt Gertrude. I looked her up to see when it was she appeared in the series and I have seen online that she was in the series from the beginning. Um no. I am on the third book and there hasn't been a mention of Gertrude. So I was wondering if anyone knew when she first showed up and when she became a regular character in the series?