Your stamp is listed in the Mundifil Catalog; 7th edition from 2015 for former Portuguese colonies (which is the current catalog). The 3 avo, the bisect, is #130a, with a catalog value of 80 Euros used. I have two of them in my Macau collection; on piece like yours; with Macau circular date cancels from 1910.
perf12: if I was to make a guess, I would consider the eBay-UK cover you posted as being legitimate. It does have a Hong Kong receiving backstamp. If I lived in the UK, I would buy it, but also get a certificate of authenticity.
The catalog is the former Afinsa catalog with the old Afinsa numbers. It is now edited by renowned Portuguese stamp expert Sr. J. Miranda da Mota. I bought a copy in September, when I was in Lisbon, from Sr. Joaquim Macas. The catalog also may be available from D & P Stamps in Chula Vista, CA.
Web sites:
http://www.filatelia.com.pt http://DPStamps.com . For information about the International Society for Portuguese Philately:
http://portugalstamps.com Steve
NOTE: I clicked on the links to make sure they work. The one for Macas does not, which is strange as I have it bookmarked and visited the site earlier this AM. Try entering it separately into your browser, with www instead of http://. For unknown reason, the SCF system changed my www to the http:// for each site.
***Fixed Link***