You won't really have a smooth transition unless the end of your shank abuts the end of the receiver (actually, the lead pipe -- see below) so that there is no "step" between the end of the shank and the receiver wall. So I'm highly skeptical that your Sterling has a smooth transition -- or that you would be able to see this by looking with aid of a torch (which I take it to be British for "flashlight" and not something like a Butane torch ). So my guess is that you didn't look down through the mouthpiece as it was inserted in the receiver and determine that the transition from end of shank to receiver/lead pipe was smooth.
There's no way to construct a universal receiver so that it would provide a smooth transition of this sort for every mouthpiece shank (because of differences in diameter and wall thickness). (I seem to recall that some Monette instruments are made with an integral -- non-removeable -- mouthpiece?) So there WILL be some sort of step or abrupt transition from the shank to the lead pipe. Unless your Sterling has it's receiver as an integral part of the lead pipe? Try very carefully inserting something like a small dowel or plastic straw or rod down your receiver and you will probably feel a lip where it joins the lead pipe.
In addition, every receiver I know of on euphoniums and tubas, etc. is soldered onto the end of the lead pipe. So the end of the lead pipe fits INTO the receiver and is then soldered. This means that there are (at least generally) TWO non-smooth transitions: shank end to receiver, and receiver to lead pipe. Thus a "gap".
The adjustable gap receiver ... er ... adjusts this gap. (Trombones are a little different since the receiver is -- generally, modulo some King ones -- integral with the rest of the lead pipe, but there's then a step transition from the end of the lead pipe to the inner slide.)
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)