Have done a little research into differences between U2 locomotives.
All the Glasgow built locos, 74,75,76,77,78,82,83 had the pipe in question. None of the Belfast built locos seem to have had it. All the Glasgow locos had Fowler style chimneys, the Belfast locos had a mixture of Fowler and Stanier chimneys. The Glasgow locos all had Fowler tenders, the Belfast locos had a mixture of Fowler and 'Breadcart' tenders. Some of the Belfast locos carried jacks on the running board next to the smokebox, but none of the Glasgow locos had them. The cab roofs were mentioned earlier, Glasgow locos had the rear corners cut away, Belfast locos had square corners.
Do bear in mind that the photographs consulted were a moment in time, and details like chimneys and tenders could have been different before or after the picture was taken.
The best thing, when modelling a particular loco, is to consult pictures of it. At least your model will be correct for the moment the picture was taken.