Brazil Greeting / Give-Away #7

Do you want the postcard from Brazil below?
Brazil greeting / give away #7:



Answer this question to my satisfaction and the postcard is yours:

Brazil is a popular area for stamp collectors. However Brazil is renowned for having issued a particular series causing frustration and head-ache to many collectors . Which series is on my mind and please give some details as to what's the challenge.

Please note
Members must have 50 posts to participate. Please allow others to answer if you have already got a postcard.

Nobody collecting Brazil?

Last chance to win these stamps on the postcard, last day today Dec 2nd ( I'm leaving tomorrow and must get them in a mailbox before then)

Jon, would that be the Mercury Commerce issue starting in 1918 - with all them watermarks?

Brian, off course it is... the postcard is yours.
Would you mind to send me you street address?

Thanks Jon! The postcard arrived last week and the cancel is superb. Once again, thanks for putting up these contests!

Thanks Brian,
I am glad it arrived and that you appreciated the postmark. It did take some effort and linguistic challenges to explain to the postal clerks that I wanted to use the hammer myself, and that the card should not be sent through the automatic cancelling/blackening machine! The clerks in there all thought I was a lunatic, nevertheless they were very helpful after 5 minutes of desperate body-language (I do not speak much Portuguese...)