Not factory though. It was a dealer item, and an ugly one at that. Looks like the generic overflows you can buy from many vendors. I'm using one of Frank McIntyre's 70-72 overflows. You can also probably find a yellow, dirty used one for sale pretty cheap.
Frank, Can you explain that one to me. Was this a dealer install item, that could be done later. Meaning after 1970? Or were there parts available before that?
Buick Dealer Service Information Bulletin 69-I-104 Subject: Radiator Coolant Reservoir for Past Model Usage- 1968, 1969 43-44-45-46-48000 Series. This bulletin is being issues in conjunction with the P and A Product Information Bulletin, #69-35, File Group 1.219, Dated December 4, 1969. Requests have been received regarding the installation of the 1970 Radiator Coolant Reservoir on past model cars. Then it shows part numbers and instructions. Frank
Dealer installed, after the fact, if requested by the owner, I'm sure. Or you could probably buy it over the counter and install yourself. Regardless, no 68/69 GS/Skylark came out of the plant with one. The dealer could install the ugly square-ish optional tank & mtg bracket available in 68/69, or when the 70+ tank was available, could retro-fit the newer one.