Just a small detail, but raises questions to me.I found rubber fragments on the frame of my restoration project, located under the seat. Tried to cut one from 3mm rubber sheet, but what is it damping?Part number 276 14747-00Does it hold cables, or damping the seat, when wide open?
Part 42 in this diagramm    www.cmsnl.com/yamaha-dt250-1976-usa_mode.../partslist/D-01.html
Thanks for your ideas.
imgur.com/UIxdqhX
 

This is the best 250B under seat photo I have in my archives and I’m not seeing it anywhere. I will check under the seat on both of mine later today and see if they have one. Both of mine are unrestored so they may still be there.

imgur.com/a/IMJHSJs

Both of mine still have that rubber damper and it is what you assumed. The rim of the seat pinches that rubber band when the seat is fully open. The band slides to the right or left along the frame tube.

imgur.com/a/YcpISWC

3 photos to scroll thru

None of my bikes have it, part diagram calls it a muffler damper? 

Muffler damper?
The muffler is on the opposite side of the bike.

"Muffler damper?
The muffler is on the opposite side of the bike."

It is a part that was first used on an earlier bike, the 1969 HT1 (model code 276) and this is likely the function on that bike. Yamaha often didn't change the name of a part, even if it was used for a different function on a later bike. 

Alan beat me to it, I just typed a similar answer, it was used to dampen muffler on earlier models like my LT2.