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. "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) (Fig.1 leftmost) as from November 11 1939 (Pelikan Schreibgeräte, Pelikan-guide).

Thirdly, the black synthetic plastic 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 synthetic plastic seal⁽¹⁾. Turning knob is smooth⁽²⁾(Fig.3). CN nib is gold-plated⁽³⁾. Judging from information above, it may be the production of 1939 or later.

Under the strained situation of WWII, Günter Wagner innovated the injection molded acrylic plastic body in early 1940's. It was used for model 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 plastic (Pelikan-collectibles).

In 1942, the production of IBIS was interrupted (Pelikan-collectibles) prior to model 100 and 100N.

Note;⁽¹⁾Although not specified in the sources, the year of its introduction seems to be early 1940's. Some 100 with Pd nib and IBIS have both black synthetic plastic seal and old knurled turning knob. It may further date back to 1939.
⁽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.