Jason,
with such title please mind that first US APFSDS (experimental, but counts) were those for 120 mm Delta gun. Two of them, Model 6 and 7, were tested before march 1962, two more, Model 9 and 10 were mentioned in report from March 1963. There were also Model 11 and 12, tried before November 1963, and Model 13 before June 1964. All those designs were 120/40 mm (latter is calibre of subprojectile), with very interesting cut cup pusher sabot, and were quite stubby (l/d 8-9:1).
In Model 14 (ante June 1964) developers returned to ring sabot. Did not saw any mention about Model 15, but Model 16 got new subprojectile, much longer than older designs (l/d 14:1), 32 mm diameter. My guess, it was that design that latter XM578 and finally XM735 were developed from.
Generally, there were about 20 Models of Delta APFSDS, Model 1 and 2 of 90/40 mm, Model 3, 4, 8 (and maybe 5?) of 104/40 mm calibre. The rest were made for 120 mm gun (or rather guns, since there is mentioned guns with smoothbore and shallow rifling tubes). Unfortunately, I have no knowledge if Model X is full designation. For example, 105/40 mm projectile T346E17 for T210 gun got it`s Model 1 and 5. In that case Model or "Mod." was a part of full designation.
So, chronologically, history of US APFSDS projectiles for 120 mm guns started in early 1960s and restarted in late 1970s/early 1980s.
Back on topic, German 120 mm TPFSDS was advertised as a part of Rheinmetall gun system, AFAIR, from the start. Guess they developed it in early 1970s, paralely with "hot" projectile. Must check notes. Saw one mention about 120 mm TPFSDS designated as DM28, which implies it was earlier specimen than DM38, but I found that "source" not very reliable.