I dont know much (in fact, anything) about Merlins, however it doesnt look like a crack to me......
It lloks more like a step between different thicknesses of the tube... either solid and machined (doubt it), or bonded on in some way
A crack would have a great deal of trouble being so perfectly straight
Also, and I know this isnt conclusive in any way, but if you line a straight edge up againts the front line of the seat tube, the area above the line in question is considerably thicker, with a taper towards the bottom
G
It lloks more like a step between different thicknesses of the tube... either solid and machined (doubt it), or bonded on in some way
A crack would have a great deal of trouble being so perfectly straight
Also, and I know this isnt conclusive in any way, but if you line a straight edge up againts the front line of the seat tube, the area above the line in question is considerably thicker, with a taper towards the bottom
G