What’s this weird dimple on older lite speed head tubes?

So I bought a late 1998 lite speed ultimate recently. Beautiful bike with an awesome full dura ace 7700 group. I am in the process of modernizing the bike a tad, with a threadless carbon fork, carbon aero wheels, 50/34 dura ace crank, etc. One thing I noticed is it has this weird dimple behind the head tube. I thought maybe my bike was defective and was repaired but after looking at other 90s lite speeds on eBay they have this too. Is it for extra strength or something?