My Mk1 is the same and it has caused some fitting problems. My engine is an MG Metro 1275 and it came with a single outlet 1275 non-MG exhaust manifold fitted, the outlet of which is too big for the down pipe of the exhaust. I have had to get a 998 manifold which fits but is awkward to join up with the down pipe because a 1275 engine is slightly taller than a 998 so the pipe is too short
The sealing problems that this caused have been overcome (for the moment) by using a sealant meant for stoves etc and which cost just over £20 for the tube
From all that, I presume that the little exhaust (which might originally come from a Hillman Imp) is meant for a 998 installation.
I recently bought the stainless version you mention on ebay so it's ready to fit if the other one blows again. I'm leaving the wee fella on meantime because I know that it's quiet so it's fine for IVA and I don't know how the loud the stainless one is but I hope it's a bit sportier!