> Did Safe C++ ever have a full, correct, fully compliant, reference implementation, or was there only (closed-source) Circle as some kind of reference implementation?

Technically speaking the clauses on either side of the "or" aren't mutually exclusive. You can have a "full, correct, fully compliant, reference implementation" that is also a closed-source implementation!

Well, unless the implication that Circle isn't "full, correct, [and] fully compliant", in which case I feel I should ask "with respect to what?" and "why do you need those requirements?"

Most languages including C and C++, had leading closed source implementations, that is why being standardised by ISO mattered.