Neat question! I went to my boss, George; our Chief Engineer and resident gearbox historian. So I asked your question and this is what he said
"The grooves were cut on a lathe very early in the blanking process; most likely BEFORE the splines are formed. The reason the splines are marked for the ratio? Its a "gross" versus "fine" scenario. I can be 30ft away and see that mark. I don't have to re-orient the transmission or myself to see it. Back when they were new, these gearboxes were shipped and stored in nested packaging, the only thing you can really see in some cases; is the input. And they may have been shipped in mixed lots, stacked 6 high and even stored on a rack 30ft off the ground.
To a lesser extent, its an easy doublecheck for the assembly tech; both on the gearbox line and the "marriage" line.
Some gearboxes have the marking grooves across all the gears, 1st, 2nd,3rd, input and cluster. This way the assembly tech and the service tech can save time identifying the specific part they need without stopping to consult a book 'Marty I need a 3 ring cluster for a 72 AMC T10.' Tags fall off cases and sometimes the ratios have been changed but the tag doesn't follow- just like axle gears; trust but verify!"
Thanks for the great question! - GE Sollish
On that note, how did that identifier come to be anyway? What process of production made them add the lines? It's an extra step in the process and I would have thought some sort of ID tag would have been more suited for identification.
