Iv'e never seen this set up before.Is it factory?Should be a fun project.

Google SRAM Dual Drive...

What's the bike?

I don't know if it came like that, but assuming you have a two-speed derailleur and two cogs on the hub, spaced 2T apart (like a 21 and a 23) it should be a good system. Eight evenly spaced gears, and slightly wider range than the 4-sp alone.

I had that same setup on my Fothergill for a while but the hub had problems, so switched to a 3-sp hub for 6 gears.

Hmmm.

That's a '50s Cyclo RD, I do believe. What's the date/model on the SA hub?

Not sure yet.I'm meeting up with the seller in a little.

Edit; Seller is thinking the bike is 1960's.

I can't tell from the picture how many cogs are on there, but there was a kit for a 3 speed cluster that would fit on an SA snap ring, while the really old hubs where ISO threaded and there was enough space on the frame for any 3 speed cluster or freewheel to fit.

Mens Schwinn Racer.

This was not offered as a stock option but for many decades Cyclo Benelux offered a conversion kit for SA hubs to allow 2 or 3 drive cogs to expand the range.

The SA hub being used would require that the longer axle was used to handle a wider block and spacers are required to give everything the right clearance and make sure the wheel is centered.

Am not sure if this bike is a 4 speed or just has a 4 speed shifter on a three speed hub and appears to be Schwinn like.

A modern variation on this is to use 2 3/32 dished cogs and a modern derailleur to provide a dual drive system and I have used this quite a number of times... it works really well when you convert a modern bike to three speed as the other drive parts are also 3/32.