The original 780 cfm 3310 was stock on Z16 chevelles, that makes it being built as early as 1965. It will have little tabs on the secondary boosters for mixture distribution for the original engine's intake manifold. I believe they were made up until 1969. They also have downleg boosters and rear metering blocks. The 3310-1 is an aftermarket replacement carb and should have the same configuration. They were made into the mid-70s.
That makes both the 780s with rear metering blocks to be 39 years old or older.

Unless you are trying to get "period correct" I'd say chasing after a 3310 or 3310-1 is foolish. In all the years that it survived, it could have been screwed up in so many ways. Also, Holley has improved the metering over the years.

One option for you is to build your own out of a Proform center. It has a lot of improvements and is much closer to what a high performance carb should be, with smoothed intake area, screw-in air bleeds, etc. I'd also say go with new metering blocks, the Holley blocks are notorious for warping or finding other ways to be unsquare.

By the time you price these out, a Quick Fuel or other aftermarket HP carb looks quite good.

If you can't afford that, buy the best model 4160 3310 you can find and substitute an aftermarket rear metering plate that uses rel jets, or else convert to a rear metering block.

The modern Holley version of the 3310 is the 80508, which from what I hear runs pretty good out of the box, and probably does a lot better job of fuel metering. Holley does make a "nostalgic" 3310, if one has to have the number stamped into the carb.

I substituted a 570 Street Avenger for an 1850 600 cfm carb and the differentce is very apparent, in a good way.

Another approach is the brand new Barry Grant Street Demon, you can now get it as a 750, with a plastic fuel bowl. It is supposed to be even better than the Street Avengers, even after some tuning.

But the bottom line is, unless one is a carb expert, one is almost always better getting a new(er) carb.

R.