I ordered interior dye from several different suppliers based off the interior color code, as well as having it custom mixed locally, and finally settled on the color closest matching my original samples. SEM NAPA RED is a match for 1973 (and 75) Dark Oxblood.
I should mention that the locally mixed color was dead on, but after it started peeling off the seat backs, I called them and found out they cannot make dye, just paint. I also plan on ordering the OE fabric from SMS and having the seat covers made by PUI, but it will be 3 months before they can do it, so I am going to use these dyed seat covers until then.