Man pages matching name xmcvtxmstringtobytestream in section 3:

XmCvtXmStringToByteStream.3.en	debian 10 / libmotif-dev
XmCvtXmStringToByteStream.3.en	debian 11 / libmotif-dev
XmCvtXmStringToByteStream.3.en	debian 12 / libmotif-dev