60's Nautique visual inspection port

I was at the Mount Dora antique boat show a couple weeks ago and they had three either 60's or early 70's Mint Ski Nautiques lined up. Each of the boats had a inspection window above where I would assume the gas tank was, center of the top of the transom where a lifting eye would be. My question is did these used to have a manual fuel gauge that you could see through this? We looked in them and didn't notice anything. The boats all had fuel gauges on the dash. This has been bugging me for two weeks can someone tell me what those are for?

Thanks

Several of us had that conversation while we were there. Out of all the boats of the same vintage mine does not have that sight glass. It does have the markings from the mold that show where to drill the hole for it and the mounting screws. The only explanation is the fact that the SS model has a different dash layout and came with a fuel gauge as standard. It did have the light back under the deck though.

Here you go Peter,very hard to see,was best without a flash. I could
if wanted highlight them with a pencil,but if you were standing
behind the boat there are marks at 12,3,6&9 and one in their exact
center