The version is JUNOS 11.4R1.6What's your current JunOS version? v17.3 and newer boot seq stopping keys is CTRL+C
Also, another tip from here:
I had already read these suggestions, I tried about ten times with various software and various methods, space bar pressed fixed or pressed repeatedly. I've always started pressing it before it appears
"Hit [Enter] to boot immediately, or space bar for command prompt"
I have never been able to access the "command prompt".