After searching the thread
(using "Santo"), I didn't find this one mentioned, but in a Pac-Man-esque fashion, there is a blue streak being chased by the 'C' on Ron's hat on the
1972 Topps #555 Ron Santo card. It's easy to find and there's no doubt in my mind that it's already on people's E&V checklists. In fact, someone on eBay actually has "Blue Slash" in his title.
The 'regular' card is on the left, and the two on the right have said blue streaks running off of his hat into the distance...
• But here's where it gets somewhat coolish and is the actual reason for this post.
In one of my 1972 Topps binder sets, the Santo has a gnarly neon-green streak passing under his nose
(probably the 'same' streak as the hat one, but located differently). I have always been on the lookout to find another example of
"Santogreenmustachio," but have never had any luck in doing so. Figured mine was just a one and done recurriation.
(Here's where people will chime in to say, "Wait, I have three Santos and they're ALL the green mustache versions." 
)
(Side note: I actually prefer the spelling of "moustache," so it's odd that I'm using the other version here. Oh well.)
Denouement time. But the other day, out of nowhere, everything changed. I finally spotted another example of the card sitting forsaken inside of a PSA 6 holder...and immediately gobbled it up
(Pac-Man reference) to claim my prize and secure a double!! Now, I can finally say there's confirmation that this thing truly exists out there!!!!
(A very low-key) Woo hoo!!!!!