Letter: Whose waistcoat?

Sir: In view of the current discussion on English grammar, I was fascinated to read in Rev Fennemore's letter today:

Sitting in my waistcoat, the sub manager strongly reminded me that I was not a Mississippi gambler ...

Why was the sub-manager wearing Mr Fennemore's waistcoat?

Yours faithfully,

Michael Williams

Handsworth,

Birmingham

3 August