Please back this up. I'm happy to entertain evidence.
In the meantime, here's an example from way back in the day of Apparel Arts, courtesy of Will:
Does that look comparable to the glen plaid worn by the original poster? I don't think so. Not even close. It has a very bold overcheck. In fact, I have a hard time understanding why Will calls it a glen check to begin with. Unless the original Apparel Arts commentary says to the contrary, it could be any random checked pattern. I would never have guessed it a glen plaid.
Take a look at the glen plaid flannels issued by Harrisons, Minnis, and Fox. I consider those very archetypical. The base plaid tends to be of medium size, but of muted contrast. The overcheck, if any, tends to be pin-stripe thin. The glen plaid sold at your typical department store tends to be even more subtle. The original poster's glen plaid looks like a very regular sort of glen plaid.