Section 22. If an allegation relative to a written instrument consisting wholly or in part of writing, print or figures is necessary, it may describe such instrument by any name or designation by which it is usually known, or by the purport thereof, without setting out a copy or facsimile of the whole or of any part thereof; and no variance between such recital or description and the instrument produced at the trial shall be material, if the identity of the instrument is evident and the purport thereof is so described as not to prejudice the defendant.