The ribs being a “service block” only applies to 428’s & 427’s, since they were no longer being installed in new vehicles by the time the ribs started showing up. Like Rory said, the ribs were on regular Dearborn Iron Foundry ( DIF ) production 360’s & 390’s, along with all the FT engines in production, through the end of DIF FE block production. By the 1971 calendar year for sure the ribs were showing up.
I’ve been logging FE date codes for a little while now trying to get a better idea when some of this stuff started, watching Facebook, eBay, Craigslist, etc. as I see dates pictured. But it’s a small sample, and I don’t have anything definitive yet. Unfortunately most people don’t detail all the characteristics of their parts as thoroughly as I’d like lol. People generally don’t pay attention to where their block was cast either. It would be nice if Ford documentation surfaced, but no telling if it even exists still. I’m guessing the dates the ribs started on 427 blocks could be different than the other FE engines, since they used a unique casting pattern on at least the drivers side & rear, so they may not have been phased in at the same time. And no telling if all the molds changed at the same time, or were phased in as old ones were replaced, or on a timetable, etc.
I’m also interested on when the Cleveland Foundry ( CF ) phased out casting FE blocks, but most don’t even realize FE parts were made there.