I have a B1/B2 10 yr multiple entry visa with the annotation " organisation xy, city, state - voluntary service summer 2005". I used the visa for that.
After this initial volunteer service (visa stamped as B2), I had once difficulties entering because someone advised me to use the B2 visa, even though I wanted to visit as a tourist and for 6 weeks only. I ended up in a very uncomfortable interview which luckily convinced the officer that I wasn't gonna marry, or work during my stay. My visa was used again, and under the stamp it it sais "viopp" -(Visa In Old PassPort, I assume?). I have returned several times since then on a visa waiver program.
Now I would like to go back to the same organisation to volunteer there again for 5 months. Will I be able to reuse that same visa with a new invitation from the same organisation (NGO)? I am planning to bring reasonable proof of return plans as well.