As far as I know high strength rivets are supplied in the "W" condition and kept in low temperatures to avoid natural aging. Then After riveting they are naturally aged. If I am not mistaken in the 6061 they will be naturally aged to T4.
A direct quote for MIL-HDBK-5J: "T6 solution heat-treated and artificially aged. Applies to products which are not cold worked after solution heat-treatment or in which the effect of cold work in flattening or straightening may not be recognized in mechanical property limits."
"T4 solution heat-treated and naturally aged to a substantially stable condition. Applies to products which are not cold worked after solution heat-treatment, or in which the effect of cold work in flattening or straightening may not be recognized in mechanical property limits."