Can a forged portion of a letter written by another person be forgery within the meaning of section 470 of the California Criminal Code?

In People v. McKenna (1938) 11 Cal.2d 327, 79 P.2d 1065, the defendant forged a portion of a letter written by another and made a photostat of an assignment of mortgage, by photographing parts of several writings and signatures and affixed them to the assignment. To the contention that these documents could not be the subject of forgery within the meaning of section 470, the court in upholding the conviction for forgery, said (p. 332, 79 P.2d p. 1068): 'The crime of forgery consists either in the false making or alteration of a document without authority or the uttering (making use) of such a document with intent to defraud. Sec. 470, Pen.Code. Whether the forged instrument is one of a particular name or character or, if genuine, would create legal liability, is immaterial; the test is whether upon its face it will have the effect of defrauding one who acts upon it as genuine. [Citations.]'