Quote:
This stamp was part of the GB 1934-36 definitives. SG446
Yes, and the only official overprint on this stamp is Morocco Agencies SG221, overprinted "MOROCCO / AGENCIES // 50 / CENTIMES" for use in the French protectorate of Morocco, issued 15-9-1936.

Conclusion: it cannot be an inverted official overprint, as the only one that exists has a different type of overprint.