slackpkg not finding an installed pkg ?

Recently when trying to record a file in MP4 format with SSR, I was compelled to install x264.

which I did in the following way:
Code:root@fujieq:~# installpkg /home/arthur/Downloads/x264-20211120-x86_64-1alien.txz Verifying package x264-20211120-x86_64-1alien.txz.
Installing package x264-20211120-x86_64-1alien.txz:
PACKAGE DESCRIPTION:
# x264 (a free h264/avc encoder)
#
# x264 is a free library for encoding H264/AVC video streams.
# The code is written from scratch.
# This package includes a commandline H.264 encoder.
#
# x264 home: http://www.videolan.org/developers/x264.html
#
Executing install script for x264-20211120-x86_64-1alien.txz.
Package x264-20211120-x86_64-1alien.txz installed.
while trying to solve that problem which was solved in https://www.linuxquestions.org/quest...64-4175732980/

I then gof this
Code:root@fujieq:~# slackpkg search x264

Looking for x264 in package list. Please wait... DONE

No package name matches the pattern.

Also found in SBo (download it with 'slackpkg download <package>'):

[ package ]
msx264-1.5.3-sbo-15.0
x264-git_20210613_5db6aa6c-sbo-15.0I think there is some contradiction here, but my experience says that I am probably missing something simple here.

Can anyone throw some light into this ?