I know this has been "known" since at least 1979 with Donat's Treblinka book, but there was a "Circle of Former Treblinka Inmates" in 1945.
Donat dated this photo to 1944, but it could be 1945. If it's 1944, then it's at least a second group reunion of these people showing a relationship that lasted more than half a year at least.

Standing from left to right: Rosenthal, Brothandel, Shimon Goldberg, Chaim Ciechanowski, Wolf Schneiderman.
Sitting from left to right: Jacob Domb, Gustav Boraks, Oscar Strawczynski, Samuel Rajsman, Arie Kudlik, Shmuel Goldberg, Lejzer Ciechanowski.
Many of these people were interrogated by the Soviets in August-October 1944 as part of several investigations.
More mentions of this circle and its activities:
21 January 1945: the Central Jewish Historical Commission (CZKH) organized a meeting in Lublin of Treblinka survivors. Survivors Samuel Razjman and Stanislaw Kohn were part of the committee, along with Philip Friedman (not a Treblinka survivor but founder of the CZKH and important elsewhere).
April 1945: a group of 35 Treblinka survivors contacted the Lodz branch of the Association of Jewish Partisans in Poland. They wanted to be recognized as partisan veterans, but the Association wouldn't accept them as an entire group since it was against their rules. Stanisław Kohn and Kalman Potasz represented the group of Treblinka survivors.
Summer 1945: the "Circle of Former Treblinka Inmates" gathered in Lodz and assisted CZKH in researching and collecting testimony about Treblinka.
15 July 1945: over a dozen Treblinka II survivors met at Oskar Strawczynski's home with Rachel Auerbach (Oyneg Shabes) and Janina Bukolska/Bucholc (Zegota) to record their testimony as a group. They practiced a "collective testimony" methodology where they were read existing testimonies in order to corroborate each other's narratives (i.e., "convergence of evidence" and "independent testimony"). Attendees below:
- Jankiel Wiernik
- Szymon Goldberg
- Sabina (Sonia) Grabińska
- Henryk Rajchman
- Aharon Czechowicz
- Henoch Brener
- Arie Kudlik
- Samuel Rajzman
- Abe Kohn
- Chaim Ciechanowski
- Samuel Willenberg
- Lejzer Ciechanowski
- Henryk Pozwolski
- Oskar Strawczyński
- Wolf Sznajdman
Fall 1945: many of them then gave testimony to the Polish Judge Łukaszkiewicz.In order to avoid the defects of excessive subjectivity in the testimonies, the various historical commissions attempted to organize collective testimony. Where several people from the same town or who had survived the same concentration camp lived together, they were brought together. The existing testimonies or chronicles of their town were read to them, and they were asked for additional information, corrections, etc. During such collective treatment of certain topics, a higher level of objectivity was very often (not always) achieved than that attained during individual interviews. Certain intimate experiences that cannot be touched on during a collective interview can be completed in individual discussions.
- Document 11 in Khurbn-Forshung. PDF
Several also published books, gave statements that are in archives in Poland/Israel/USA, participated in later trials, etc.
I don't think the names of all 35 are known or collected in one place, along with when/where they left their mark. Many of the relevant early documents are in the JHI archives. Those have had a "temporary loss of access" for 7-8 months now, so we'll see how long "temporary" turns out to be. [edit: a book published in Polish has an article that might have all 35 names -- or at least the author of one of the articles in the book names 35 people who left early testimonies.]
If I can find more on this Circle and what they did, I might create a Wiki page.
edit: found more and will add here... but not organized enough for a Wiki page yet.