I like an old Wulff TT 6/7 on mine. But, try different lines to see what works for you.

Some of the System rods appear to have reel seats meant to use a factory fighting butt and some don't.

I've had bunch of System rods, but never had my hands on one with the actual fighting butt.

This System 10 reel seat is made for a fighting butt.



This particular System 6 isn't.



I've seen rods of the same weights with different reel seats. Don't know if it was an option or rods made in a different time period.

I retro-fitted a Struble fighting butt on System 7 rod with the rubber plug.
I simply used a thin sheet of rubber for a shim, and it fishes great.

Those reel seats were from an OEM supplier, Allan or Varmac, probably, in a more or less standard configuration. They were designed to be assembled according to the wishes of a hobbyist or volume-buyer for a production series. The absence of a premounted plug or cap enabled them to be used in both up and down locking configuration and positioned relative to the end of the blank--possibly extending somewhat--according to the builder's wishes or rod design. Then a rubber plug or cap was fitted, possibly customized to the company, as in the rubber plugs with Fenwick logo, or just in a nonstandard color. Caps might be engraved with a logo or model info.

In other words, this was a versatile OEM style that could be sourced and adapted to a maker's purpose. Especially in 6 or 7 weights and under, the plug or cap style isn't really an indication that rods with these seats were provided with a fighting butt or that a specific accessory was available. But the user could remove the cap or plug to install an easily-made fighting butt.

As for lines, the SA "System" product line was an early marketing package for no-brainer tackle selection, and they had a matching reel-size and line for each rod model. SA lines of the time were state-of-the-art to AFTMA standards. The System rods would match with a DT of the #ed weight, or that plus one in WF. More specialized lines are more a product of the graphite era and some new techniques that came with stiffer fly poles, plus the headscratching mode that purports to enable more fish catching.

But for an SA System 6, start with a DT 6 or WF7 of glass era configuration. The SA Aircel or Supreme would have been the choice of the time, and other brands would work just as well if the user preferred their texture, minor differences in taper, and so on. The, Cortalnd 333 or 444 were also go-to lines. And many house brands were very similar, produced by these same two makers and sold at lower cost. Today, the 406 lines, designed with fiberglass rods in mind, and with the era when lines were perfected for the type, might be an easy choice. But if you read the product description,there are plenty of conventional DT and WF lines available, often relatively inexpensive because they are for fishing, not scratching the head and holding the tongue a certain way.

That is a great 6 weight, among the best ever produced, then or now. Keep it simple with line choice and the rod will show you.

In my searches recently I've seen several rods with this same fighting butt. Interesting that they all seemed to have the dark reel seat which doesn't appear to have been meant for a fighting butt.

Also, contacted SA in hopes they might have some new/old stock. No luck there. Be nice to know if any catalogs from that era ever offered a fighting butt.

5 or 6wt Rio gold would be good.

Tagging along on this thread, I also have a system 6. Any recommendations for a WF 7 salt water line, both floating and intermediate?