+1, the alpha-numeric code is a year-month code, with the OP's example being from April 1986. A steering column with six, spiral ridges is indicative of CrMo. While there is a hierarchy from hi-tensile to Mangalloy (carbon-manganese) to CrMo (chromium molybdenum) to Prestige (heat treated CrMo) there wan't the same variation as between tubesets. For instance, in the CrMo road forks, Tange #1 through #5 , 900, 1000 and Infinity all used the same 0.9mm wall thickness. While there were round and oval versions, in two different diameters, but this was to fit the sockets of the various crown options, which were designed for different applications (i.e, road, track, touring). The rake would also vary depending on the application, but this was often customer specific. So, when you're talking 1980s Tange road forks, there isn't much of a hierarchy, with the variation being primarily alloy, application and cosmetic style.