37901 to 37904 original roof conversion. This consists of a single printed roof panel with all the detail incorporated. This version has the twin exhaust ports. As fitted from the original conversion in 1986 until the early 2000's. 
N.B 37904 needs 5thou plasticard strips at 1.5mm wide so the cantrail grilles are split it more sections 8 in total.
To fit the exhaust panel a large hole needs to be cut into the roof starting at the rib at the fan end of the loco to 1mm before the rib of the exhaust which has been removed from the model (see the last photo). The print is angled so it sits in the hole with the sides slightly beneath the side rib line.