Just to clarify a possible misunderstanding, 1986's 600 SIS was part of New 600EX, which had started out as friction only in the 1984 model year. In the 1986 model year it was upgraded with a SIS option consisting of the new RD-6208 rear derailleur, SL-6208 shift levers and MF-6208 freewheel. Besides SIS compatibility, the big changes for the rear derailleur was the incorporation of a slant parallelogram and cable tension adjuster. Even though the SIS levers had a friction mode, Shimano continued to offer the friction version of the levers and rear derailleur for manufacturers who were not convinced of indexed shifting and/or wanted to economize. Attached is the relevant section of the 1986 catalog page, showing the availability of both versions,so you can see the aesthetic differences. Before anybody asks, Shimano never put the "New" or "EX" designations on the actual product..1986 was also the year that Shimano literature dropped the "New" from New 600EX, reverting to just 600EX, though I still call it New 600EX to distinguish it from earlier 600EX (aka Arabesque).

Besides New 600EX, there were some other non-indexed Shimano derailleurs newly released for the 1984 model year. Most notable is Deore XT, but there were also some Light Action and/or Z-series introduced that year. There were also some new Light Action derailleurs released for the 1986 model year that were not listed a SIS compatible, though the designs certainly look SIS compatible. For instance, the 1986 RD-L525 is available in three different cage lengths but only the shortest cage version is listed as being SIS compatible. That's very perplexing.

As noted by member dddd, the shift lever mechanism can "gum up" . Also, some of the early down tube shift levers suffer from brittle failure of the indexing tabs. However, I believe that the stem sifters are similar to the thumb shifters, in that they are semi-sealed units, unlike the down tube shift levers.

Regarding the history and model name syntax, you'll just have to live with the ongoing discussion, as you're one who opened those cans.

I have the Colnago master with DA7400(SL-7400) which can be run as SIS or friction. So far I had no problems at all, and it still runs real smooth. I do regular maintenance or occasional adjustment for sure. If you have your model number, you can go to Shimano web site and look for service documentation on it. Very simple to compare todays shifting system but very effective imho.

This is link to the search page for the manuals and documents. Hope that helps.

si.shimano.com/#/

Finally confirmed that my Ross is a 1989. They offered the SIS in a couple of different configurations.

I have some shifters like the ones shown on the stem. The rear derailleur lever is indexing or not, your choice, I always use indexing. The front derailleur lever is not indexed. They are 6 speed. They were originally thumb shifters on a cheap MTB. I have then mounted on the stem just like the photo. They have been on my nicest beater bike since the late 1990's. They have about 10,000 miles on them. They have been in a few downpours, never any problems. They are much more reliable than my modern brifter bikes.,
RX100, DA 9 speed triple, 10 speed 105.

A shift cable started to fray in the shifter around 6,000 miles. I see that as normal maintenance, a little better than the others.