An overextended metaphor

I love double-entendres. It must be something to do with growing up with Carry-On films, but I can't resist their earthy smut.
So let me overextend a metaphor for you, if I may...

When I got home yesterday, I noticed something as soon as I came.
My knob was looking a bit worn, and my wife agreed.
She said it felt all rough and had lost its shine.
She told me about this old man's knob she had seen on the market.
I rushed down and grabbed it as soon as I saw it.
"It's my only one" he declared.
It looked well used, but I just had to have it. It was so big.
"How much?" I asked, tossing it in my hand, as it gave a satisfying slap in my palm.
"Four quid."
The price was Okay, but I wasn't going to quibble. My own knob looked a bit small in comparison and tended to stick, so I agreed.
Later, I nailed it to the front door for all to admire, and after a bit of a rub to make it stand out proud, it was ready to use.
It gave me real pleasure every time someone pulled on it when they wanted to enter.
My knob is always proudly on display, for all to see and use, especially (but not exclusively) by my wife.