So I checked the pics of the US Hei Hei from 97 that sold the other week. It had routing on the downtube as pointed out and the grey stickers. The SN was prefixed with K then 19 for the size then the number of the item in the run. This hasn't got a K so it looks more and more like a mid season 97 or 98 model.
I don't see any reason to doubt that it's a 1997 Hei Hei.
They went over to all top tube routing during the 97 production run - it seems as though roughly a third to a half of 97 Hei Heis and KKs were tt.
The decals for 97 were either blue (as in the Explosif etc) or yellow (as in the Kilauea etc). For Hei Heis and KKs they used both/either. I don't know why the yellow ones on this example have a blue underlay, but it wouldn't give me any concerns as to whether they are original.
The serial number doesn't point definitively to 1997, but it is consistent with 1997. B191059 means either the 1,059th size 19 Hei Hei or the 1,059th size 19 of any brand that TST had made, I don't know which. (Most people believe the former, but I find it hard to credit that they were making that many). I know that B19997 was a 96 Hei Hei, so this one was 62 frames later. It was during 1997 that they started using a K prefix for King Kahunas (then for 99 King Kahunas were KK and Hei Heis were KH), but 98 Hei Heis were still B. I don't think they made many Hei Heis from 98 onwards, as the demand was all for KKs.