Help With George Marshall #1289 Variants

I could use some help understanding the following Scott entry for #1289:


Most of it make sense to me. But what is the date of issue for 1289b?

I note also that in both the "a" and "b" variants, each exists with both shiny gum, and dull gum. How does this relate to the date of issue, which for "a" is said to be 1973? Did both gum variations exist from the start?

I'm expanding my horizons here in looking more closely at a stamp's variants, so anything you can add to my knowledge or understanding will help here.

Including good sources of information for more details than one finds even in Scott's U.S. "Specialized" catalog: if I wanted more detailed information about this particular stamp, would there be someplace to look?

TIA.

Basil