Hi Les,
Tradition Tubes are the narrowest of the lot.
Unless your Sentria II Tube fitted without modification, straight onto your Tradition Emptor, then the Bag-Topper will not fit. They are a fair bit narrower.
That's why most conversions involve drilling and re-sealing the Tradition Emptor to take a later, wider tube.
HEPA bags aren't available for these earlier models. That's a Manufacturer's restriction to sell the latest, greatest vacuums and bags. If they were all backwardly-compatible, no-one would pay £1,000 for an Avaliar, when a Classic would do. You won't find Gillette five-blade Turbo Razorblades to fit your old razor handle, either.
I tried the wider Legend II Emptor, Tube, and HEPA Bag assembly on my Tradition. Didn't notice any difference. But our UK Traditions run at high speed constantly. We don't have the slow=head, fast=tube, switching you have Stateside. So we have no loss of suction with the narrower tube. I suspect the narrow tube actually speeds-up airflow (venturi effect) and carries heavier particles to the bag.
If you don't have allergies, I'd suggest there's no need for HEPA (actually pseudo HEPA-Like). HEPA must reduce airflow? (smaller perforations catch smaller particles), but that's a trade-off. You get nothing for free. The airflow is restricted further still. Heavier particles must be left behind in the carpet as a result of slower air. Reminds me of the Tetley Tea Bags 'breakthrough' with finer perforations. They were using more tea dust instead of leaves!
Some say they prefer the cleaner, dust-free, Motor-Housing. I was the first to see inside mine for forty years. I don't care what will be in there by the year 2059.
One explanation offered for narrow Tradition Tubes, was that early single-ply bags burst under high pressure. No. They're constrained by a strong cloth bag. It works like a tyre innertube.
There's a fair bit of supposition, guesswork, and myth surrounding these machines, and bags. You'll need to form your own opinion and conclusions. Personally, I feel there's a lot of marketing hype here, to create a demand for disposable bags. They're losing the war to bagless machines.
I have the Tradition, Heritage, and Legend. Compared them all. I've chosen to keep mine all-original, unmodified, and stick with standard paper bags. When those run out and are no longer available, I have my own reusable inner bag which will last for ever. Just like our old Electrolux.
The best upgrade was the new Roller and Ribbed Drivebelt.
