These marks are not from the blown up gas chambers in Auschwitz II Birkenau (which remain blown-up today save the Bunker I or Red House gas chambers, where today a part of the exhibit and memorial site can be found. They are from the gas chamber in Auschwitz I Stammlager.

The first gassing in Auschwitz took place in the summer of 1941. It's main victims were at that time Russian POWs. These took place in the cellar of Block 11, the camp prison, and were of an experimental type of nature since they represent some of the first tries to kill a large number of people (about 600) with Zyklon B. While the test was successful, the cellar of Block 11 proved hard to vent after a gassing had taken place.

In light of this, the camp commander ordered a new place found for the gas chambers. This was the so-called "corpse room" in the camp crematoria building, which had previously been used as a place to execute prisoners already. This is the room where these marks are located and which can be viewed today. It was large enough, easy enough to air proof, and it already had a ventilation system, build in after members of the Gestapo had complaint about the stench after previous executions in the room by shooting. Also, because of its particular architecture it was easy for the personal to drill holes in the roof, in order to fill the room with Zyklon B.

It was never intended for permanent use and when Auschwitz II Birkenau started expanding first in January 1942 with the construction of the two Bunker gas chambers and then later in the spring of 1943 with the erection of the two big crematoria (II and III, later even more expanded to IV and V), the gas chamber in Stammlager fell out of use more and more. In December 1942 the last gassing there took place, its victims was the Jewish Sonderkommando charged with burning the corpses of the Birkenau gassing victims before the crematoria in Birkenau had been functioning properly.

After that one, the room was again used to conduct executions by shooting and other killing methods until sometime in 1944 when the building was reconfigured as an air raid shelter. The number of victims killed in this particular chamber is unclear today but most historians estimate it in the range of about 10.000.

After Auschwitz was liberated, the air raid shelter was build back into its previous state as a gas chamber / crematoria following the descriptions of prisoners who had worked there and the plans left by the Nazi administration of the camp. In this sense, what can be seen there today is both a reconstruction as well as reconstitution of the original state of things.

Now, as for the scratch marks: As far as can be ascertained from the literature on the subject, most of them appear to be original for they were left undisturbed during the Nazis' reconfiguration of the building as an air raid shelter. During that time, a few walls and implements were taken out but the walls were not painted and the wall where these were was also not taken out. So, all in all, they appear to be mostly original. Also, they are quite in line with testimony given about the gassing in general.

According to Dov Pasikowic, survivor of one of the Jewish Sonderkommandos who witnessed the gassing of the Hungarian Jews in Auschwitz in 1944: "Many women miscarried during the poisoning; people hit each other, people scratched with their nails. There were finger nail marks on people. Everyone wanted to survive but it was impossible there."

Now, it can be assumed that during the subsequent years of use as a memorial and museum site, visitors did leave markings on the walls there, so the scratches depicted in the photo might not all be marks left during the use of the room as a gas chamber. On the whole though, there is nothing that would indicate according to sources and literature that these markings are anything other than authentic / left overs from the use of the room as a gas chamber.

As for your second question:

I would welcome any information on exactly which parts of various camps are reconstructions vs. originals.

In light of the fact that the Nazis ran 42.000 camps all over Europe with 24 big Concentration Camps that again had over a thousand sub-camps, I am afraid this is too broad to answer in this forum. When it comes to gas chambers though, "authentic" gas chambers as they were used by the Nazis and with minimal restorative work done can be found in Majdanek as well as Mauthausen while those in Sachsenhausen e.g. are reconstructions.

Also, for a very good answer an in-depth answer on the issue of reconstruction, authenticity, and so forth of the gas chambers, see this answer by /u/kieslowskifan who puts it well when they write: "Crematoria I serves as a placeholder of physical memory for all of these other sites; people were murdered at Crematoria I, but not all of Auschwitz's victims died there."

Sources:

• Eugen Kogon et. al. (ed.): Nationalsozialistische Massentötugen durch Giftgas.

• Bertrand Perz et. al. (ed.): Neue Studien zu Nationalsozialistischen Massentötungen durch Giftgas.

• Nicholaus Wachsmann: KL. A history of the Nazi Concentration Camps.

• Dan Stone: The Liberation of the Camps.