Toward the end of 1930's, Günter Wagner made major changes in specifications.

Firstly, the company switched the material from ebonite to celluloid (or cellulose acetate), which occurred first in cap tubes, then cap tops, finally filling mechanisms (Pelikan History). "By 1939, ebonite was gone from Pelikan pens (Pelikan History)". I suppose the similar switch was made in IBIS; as of 1939, IBIS would be made of celluloid (or cellulose acetate) except for metal parts and the nib unit.

Secondly, the material of nibs was changed. In October 1939, the government forbade the use of palladium nibs for the domestic market. Günter Wagner was forced to use chromium/nickel (CN) - steel nib (CN nib) for IBIS for the domestic market (Fig.1 leftmost, Fig.4) as from November 11 1939 (Pelikan Schreibgeräte, Pelikan-guide). 

Thirdly, the black elastomer seal was introduced⁽¹⁾.

Fourthly, "IBIS" imprint disappeared from the cap tube (Pelikan-collectibles).

Fig.2 is the "NOS” IBIS. It is made of marbled celluloid with CN nib and a black elastomer seal⁽¹⁾. Turning knob is smooth⁽²⁾(Fig.3). CN nib is gold-plated⁽³⁾. Judging from the information above, it may be the production of 1939 or later.

Under the strained situation of WWII, Günter Wagner innovated the injection molded acrylic resin body in early 1940's. It was used for the models 100 and 100N. Whether the new body was used for IBIS is unknown. Fig.4 is the model IBIS with CN nib (Pelikan-collectibles). The the inner threads (Fig.4 arrow) seems to be seen through the grip section, which suggests that it is the injection molded acrylic resin (Pelikan-collectibles).

In 1942, the production of IBIS was interrupted (Pelikan-collectibles)⁽⁴⁾.

Note;⁽¹⁾Any records do not specify exactly when black elastomer seals were introduced in the IBIS. As the introduction of black elastomer seal to the model 100 is presumed to be around 1938, it is possible that their introduction dates back as early as 1938.
⁽²⁾As the delicate groove of the knurled turning knob was difficult to produce with celluloid, the turning knob became smooth (Pelikan Schreibgeräte).
⁽³⁾We can see that at least early CN nibs were gold-plated.
⁽⁴⁾The company probably reduced the number of models due to resource and labor constraints caused by the war effort.