Old 4D Maglite

[Telman2]

I'm new here and I read about upgrading a 4D Mag-Lite using a Terralux MiniStar30M-EX LED. I have a very old 4D Mag-lite that I've owned for over 25 years and thought this would be a great upgrade. I ordered the Terralux unit and find it doesn't fit my light. My question is are the older Mag-Lites different from the newer ones? I doubled checked the forum and the place I got the Terralux from and it's the right unit. The Terralux won't seat down into the head and if I just try and screw the Terralux into the bulb socket without the head in place they appear to be different sizes.