T bar was part of the Koga-ITM Trekking-bar package deal..

one option a SJS Thorn accessory bar
Thorn Accessory Bar T Shaped 55 mm Extension - £16.33
Thorn Accessory Bar MK2 T Shaped 105 mm Extension - £19.99
etc. Thorn Bicycle Equipment Shop at SJS Cycles

fitted on above the stem you have for the handlebars .. requiring a bit higher steerer tube extension obviously ..

I put a second stem beneath the bars in that picture .