7107 was in fact the first speed lettered SP unit. I was one of the first ones to see it as I worked at M-K in Boise at the time. It was part of an order of GP40 cores rebuilt to SP's specs including Dash-2 electrical cabinets/controls, making them model GP40M-2. While technically "manufactured", for accounting purposes they qualify as new units (and can be depreciated) due to the extent of rebuilding, versus just considered repairs.

The series started with SSW 7274 for the first 20 (continuing the series behind the last new EMD GP40-2), then as I recall SP exercised an option for 30 more for SP proper which ran to 7299 where they ran into the 7300 SD40R rebuilds. The balance of the order dropped back and started at 7100. I believe this also may have been the first SP units with ditch lights (or did the GP60s beat them) and as rebuilt they had air conditioning added and Santa Fe style beacons.

If you look at the first few speed lettered SP GP40M-2 units you will find there were variations in the paint design. I think the size of the side lettering changed, and the side sills had been painted white with the reflective stripe, then that was simplified to just the width of the reflective stripe.

M-K's shop in Mountain Top, PA also rebuilt a few units within this order as I recall. I'm not sure which units they did or when they shifted over to speed lettering, but Strapac's book might list them.

Last year I ran WAMX 3917 which was stenciled as a "GP39-2" but is obviously a GP35 rebuild. I was told it was an xSP unit, but it didn't look right for a SP rebuild. I noticed the BNSF 2872 still stenciled in the cab, which I realized was its BN number, and it was a SP core, but had been rebuilt into a GP39M (more accurately a GP35M-2) for BN while I was at M-K. Over the years I've run and seen a whole lot of M-K rebuilds (including some I'd seen as a teen and snagged a ride on), but it only took me 28 years to actually run one I had worked on at M-K. There was a small additional order done of BN GP39M units while I was there, so BN 2872 and SP 7107 went through the M-K shop about the same time. As rebuilt, 2872 had those tin can strobe lights which I though were pretty worthless, especially in the daylight.

Some may think that the speed lettering was the beginning of the end for the SP, but I say it was the failed SPSF merger and the removal of the gyralites. I will contend the beginning of the end was the ICC approval of the BN merger in 1970 without considering the long term consequences, especially to the Milwaukee and Rock Island. We really needed to keep three strong competitors on the west coast and on the east coast. These duopolies don't create enough competition, and everyone was fooled by the improvements in service and rates that "single line service" was to provide.