Factory 1500 TC extremely rare. Only used in the first 10 or so S1 Elans. They were pulled from cars that came in for service and replaced with 1600 as Lotus wanted the 1500s for racing.
"1500 federal engine" = either a Renault 1470 high compression, 697-04 (which was sent to the US so could be construed as "federal"); or, more likely, 1565 normal compression, 821-30 engine that was used later. Both have similar power outputs with the 1565 being slightly torquier and 1470 revving cleaner.