In T 2321/08 the board considered the question of whether R. 27(1)(b) EPC 1973 required that prior art known to the applicant be acknowledged in the application already at the time of filing it. The board came to the conclusion that R. 27(1)(b) EPC 1973, or equivalent R. 42(1)(b) EPC, does not put a stringent obligation on the applicant to acknowledge prior art known to him, and to cite documents known to him reflecting this prior art, already at the time of filing the application. Furthermore, no requirement of the EPC prohibits amending an application in order to meet the provisions set out in R. 27(1)(b) EPC 1973 or R. 42(1)(b) EPC (endorsed by T 1123/09 and cited in Guidelines F‑II, 4.3 – November 2018 version).
In T 11/82 (OJ 1983, 479) it was pointed out that the mere addition to the description of a reference to prior art could not reasonably be interpreted as the addition of "subject-matter", contrary to Art. 123(2) EPC 1973. Whether it did so would clearly depend on the actual language used and the circumstances of the case.
In T 211/83 the board even considered it essential that details of the way a component of the invention worked, which could only be inferred from previously published advertising literature referred to in the description, be included in the description since they were of significance for the invention.
In T 450/97 (OJ 1999, 67) the board confirmed that the mere addition of a reference to prior art did not contravene Art. 123(2) EPC 1973. It added that after limitation of the claims, also at the opposition stage, a document which subsequently proved not only to be the closest state of the art, but also to be essential for understanding the invention within the meaning of R. 27(1)(b) EPC 1973 was to be introduced in the amended description. See also T 276/07.
In T 889/93 the appellant (patent applicant) explained that the closest prior art was shown in two figures of the contested application. In his reply to the board's objection of obviousness he submitted that these figures did not show the state-of-the-art device correctly as they were misleadingly oversimplified. The board allowed originally filed drawings to be replaced by better ones since this only removed an inaccuracy in the representation of the state of the art and did not affect the disclosure of the invention per se. T 1039/93 found along similar lines. In the case at issue, certain figures of the application as filed had been erroneously labelled as prior art while in fact representing in-house technical knowledge of the appellant that had not been made available to the public. The board did not object to the deletion of the labels "prior art" from the relevant figures since such deletion neither infringed Art. 123(2) EPC 1973 nor affected the disclosure of the state of the art; on the contrary such amendment was necessary to avoid giving an inaccurate representation of the state of the art. After the amendment the description correctly indicated the prior art as required by R. 27(1)(b) EPC 1973.