That's a late production 25-2 that has had its barrel cut to 4" along the way. Not so mysterious at all. The barrel stampings being so close to the muzzle are an immediate tip off. They should be more centered on the barrel and they were until it was shortened.
I don't like the trade unless the 5" M-25-2 is lettered as a 5" gun.
Does anyone realize this thread is 2 1/2 years old...just saying..
To the best of my knowledge no 5" 25-2s were made. If you saw one it was either modified from original or was a special order. S&W would do special orders back in the day but they are very rare and very valuable.
PS: A 5" barrel is not rare on all S&W revolvers. This was a standard barrel length on numerous models. Just not the Model 25.
The 25-5 is found with over size throats. That serial is around 1977.