With the v7.7.4 installer index file (install/index.php) it should display:
Welcome to the Upgrade/Install Utility for UBB.threads
Upgrade UBB.threads from a previous version
Version checks:
• Database structure version: 7.7.4
• Script version from database: 7.7.4
• Script version from filesystem: 7.7.4
Continuing will perform the following tasks:
• Upgrade your current version of UBB.threads to UBB.threads 7.7.4.
• Retain your current forum threads, posts, users and other information.
It should not display v7.7.3 in the version checks unless you're on 7.7.3, and v7.7.4 should display for "with the..." heading and "upgrade yoru current version to 7.7.4"