Dads Army Theme on Piano

Vintage comedy now on BBC2 with Dads Army. This tune was etched onto my brain when I was very young. I still dont see the relevance of the A21 other than it rhymes with gun.\r
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Who do you think you are kidding Mr Hitler\r
If you think old Englands done\r
We are the boys who will stop your little game\r
We are the boys who will make you think again\r
So who do you think you are kidding Mr Hitler\r
If you think old Englands done\r
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Mr Brown goes off to town on the A21\r
Then he comes home each evening \r
And hes ready with his gun\r
So who do you think...