I always assumed that Raleigh 555 was re-badged Reynolds 501, based on the following:
1. During the mid-80s when this tubing was used, Raleigh was still owned by Tube Investments, the manufacturer of Reynolds tubing.
2. As Lofter points out, Raleigh USA was still marketing Reynolds 753 and 531 frames along with Raleigh 555, so Raleigh 555 would not have been either of those two tubesets and would presumibly have been a less expensive tubeset, which was Reynolds 501's position in the Reynolds line-up.
3. Raleigh 555 was a chromium-molybdenum tubeset. Reynolds only chromium-molybdenum tubeset at time was Reynolds 531.
4. Raleigh 555 was unique to the US market. At the time Raleigh USA was using 555 tubests, Raleigh bicycles in other markets were being marketed with Reynolds 501.
5. The Raleigh 555 decal is remarkably similar to a Reynolds decal. If for some reason, Raleigh USA was using non-Reynolds tubing, I seriously doubt that Tubing Investments would have permitted the use of a decal design that consumers would associate with Reynolds tubesets.
6. There was a lighter version of both the Raleigh and Reynolds tubests, called 555SL and 501SL repectively.
Basically, Reynolds 501 was a seamed, butted tubeset intended for mid-range, sports bicycles. It was about 0.5 lbs (225 g) heavier than a Reynolds 531 tubset and about 0.9 lbs (425 g) heavier than the top of the line Reynolds 753. However, the 501SL tubeset gave away little weight advantage to 531, being only 0.16 lb (75g) heavier.
Comparable seamed, butted tubesets from other manufacturers would have been Columbus Cromor, Ishiwata EXO-M and Tange Infinity.