Just ran Win 98 Update and I am advised to install Security update JScript 5.1 for Windows 98 and NT 4.0 - Q814078.
I had this Critical Update on th 19th March via Win98 Update and it was for JScript 5.6, which I installed.
I have also confirmed my current version of jscript.dll which is 18.104.22.16813 ie the patch had taken and is still there. I only have one such file in c:\windows\system and all the file attributes agree with that given in the Microsoft Knowledge Base Article - 814078 .
I will hold fire with installing the JScript 5.1 update. A reverse Update if ever I saw one!!