It is problematic to call any tender as belonging to a specific locomotive, as they were swapped around by the Rio Grande. As a loco was shopped it might be done before its tender was ready so another would take it's place. Or maybe a loco with a worn out tender would get one from the dead engine line that was in better shape. 318 and 346 were not immune to this practice.
In the mid 1930's, 345 had a large tender with straight sides and coal boards. The upper sides of that tender's shell were missing the standard 'beading' normally found there but it was present in the rear of the tender around the air tank and water hatch. Made the car very distinctive. That tender found itself behind the 340 in the 1940's. And this pairing made its way to Montrose for use on the Ouray Branch. 318 was also assigned there and it had a tender similar to the third photo of the flared top tank from Key. Key put standard D&RGW C-class tender trucks under the car which is not correct. They should be F&CC style trucks which were on this F&CC tender.
In the late 1940's, '48 IIRC, the Cow Creek tank burned. That left Montrose, Ridgway and Ouray as water sources for the branch. That little F&CC tender held somewhat less than a normal 2,500 gallon C-class tender and it wasn't enough to reliably make the run from Montrose to Ridgway or vice versa, particularly with a heavy train. But the big straight sided tender held a couple hundred gallons more than 2,500 and made the trip without a problem. This is why, whether it is 318 or 340 that is running to Ouray after the Cow Creek fire, you will see the big tender behind the engine It is also the reason why there are quite a few shots of 340 with a disconnected (read "unused") 318 tender by the Montrose engine house and some shots of 318 with a disconnected 318 tender there also.
So how did the tenders end up where they are now? At the end of narrow gauge operations the 318 made the last run on the Ouray branch and it was using the big tender. Cornelius Hauck bought the 318 and both pieces ended up being hauled to Alamosa and deposited at the Narrow Gauge Motel next to Bob Richardson's 346. The 340 got hooked up to the little 318 tender and hauled to Buena Park, CA, to run on the Knott's Berry Farm's Ghost Town and Calico after being bought by Walter Knott for that purpose.
For years you could clearly see the raised "318" on the back of the little tender as it followed the 340 around the park. In a 1991 rebuild the tank was taken down to bare metal and the numbers are no longer visible. But the oddball tender trucks are still there.
Some time after the 346 and 318 came to Golden in 1959, the museum workers proposed firing up the 346. While setting up the engine to run they discovered that the 346's flare top tender leaked like a watering can and the decision was made to swap tenders with the 318, and the tender dance continued until the 346 got her new tender in the most recent and incredibly great rebuild of that locomotive.
So in their final(?) resting places the 318 has the 340's (345's before that) tender, 340 has the 318's F&CC tender and 346 finally has her own tender back.
