Anyone ID this Cook and Moore sketch?

[glen]

Many years ago I had a tape recording from the radio, but it may be a tv
sketch, which I pretty sure was Peter Cooke and Dudley Moore.

Maybe set on a train, not sure.

One man has a cardboard box on his knees and is pestering another man
to guess what's in the box.

"I haven't got a snake in my box ...
Cleopatra had an asp, it bit her in the bra..."

"I haven't got a fish in my box, oh no, there's no fish in my
box. If I did, the water would leak out over my knees, because they
haven't waterproofed cardboard, well if they have they haven't told me,
and if they told me, I've forgotten."

Other man: "I'm sorry but you are a bore."

"A boar, a boar, there's no boar in my box!"

Sorry that's all I can remember, any ideas?

Thanks