Classic & Vintage - Old-school Dura Ace chainring markings: 'TO', 'SR', 'SS', 'YG'

So, I've got a pile of NOS early Dura Ace chainrings, presumably from the 1970s or early 1980s (?).
Most are packaged like so, some are without packaging:

270053

Most are simply marked 'Shimano' + '[tooth count]'.

However, several have other letter-markings next to the 'Shimano' stamping: four different markings I've found are 'TO', 'SR', 'SS' and 'YG'.

Does anybody know what these designations mean? I've tried searching for info, and not much luck.

'SS' = track, maybe? I know the Shimano track cogs are currently designated as SS-7600, etc., for 'single sprocket'.
Seems vaguely possible, but not so sure about that at all. And what about the others?

These are the markings in question:

'TO':
270054

'SR':
270055

'SS'
270056

'YG' (and the 'circle-S', which I've not seen elsewhere):
270057

Any thoughts? Little help? Thanks!

As of 1976, Shimano used a 2-letter date code with the first letter being the year, starting with A=1976, and the 2nd year being the production month, beginning with A=January.
The first-letter Z came in 2001.

In 2002, the letter A started again, so in 2012 we are at letter K.

The date codes prior to 1976 are different, as the letters R and T were from around 1973 or so.
Those pre-1976 first letters may have progressed 4 times per year (3mo. periods), but I can't say for sure.
The pre-1976 second letter may have progressed in two-week periods.
These are possibilities. Does anyone else have additional info?

From the style of the rings imaged and packaging they are first generation Dura-Ace.
Shimano users to my awareness are not such date Code warriors.
Second generation inside rings appear like Super Record, and the outside rings have no inner webbing, and are just split beyond the chainring bolts, Dura-Ace EX. Similar packaging, but EX was prominent.

Yes, I suspected First Gen and/or 7000 Dura Ace. I appreciate you chiming in.

The letter-codes, in particular, struck me because I have no idea what they're about, and they only appear on four of over a dozen otherwise very similar rings I have here.

Could be some kind of date code, I suppose, but if so not one that was always in use for these chainrings given that only a couple of the rings are so marked. Most of them are otherwise identical but without the letter-codes. I do have a 'T', which was one of the date code letters mentioned used in '73 (S is right in there with T and R, too...and 4 month periods would certainly make it to 'Y' before '76). Hm. 'Preciate the info on that.

Nice solid rings, even snagged one with the chain-guard on there (pictured at top)! Not super appropriate for the mostly-mid to later 80s stuff in the parts box and on my various bikes, but I'm sure I'll find a use for them. Set for life for 3/32 rings for SS (44, 47, 48, 49...) or 'compacting' the big my big ring on 130bcd cranks, I guess.