Edo_Rajh wrote: "... Stamp Atlas and see that they listed as first Croatian stamp Obligatory Tax Stamp issued on 1st April 1991. Scott #1 for Croatia is issue from 21st November 1991, while Croatian post consider as first issue one from 9th September 1991 (air mail).
So, I'm not sure how reliable is Stamp Atlas because it seems to me that they have different definition of first issue than this club.
I don't think it is fair to say that they are not reliable; it is more a matter of definition.
If you want STRICT chronology for Croatia, you will probably say:
#1 Obligatory Tax Stamp issued on 1st April 1991
#2 Air mail issued 9th September 1991
#3 Regular postage stamp issued 21st November 1991
But the Scott catalog (and others, I believe) do not organize the issues this way. Scott separates regular mail (#1, #2, so forth) and "the rest", called "back-of-the-book", or BOB, issues. Those issues are again sub-divided into
1) semi-postals (with benefit surcharges for example (Red Cross)) as the B numbers (B1, B2,...)
2) Air mails like C1, C2 ...
and a host of other groups.
So, according to this scheme, we have:
#1 Regular postage stamp issued 21st November 1991
#C1 Air mail issued 9th September 1991
#R1 Obligatory Tax Stamp issued on 1st April 1991
(I'm not sure about the R1). Magic, they ALL became #1's!

What joy for us!
What we (FICC) do is to follow Scott in some sense, by separating the "normal" postage stamps and "all the others". The only "mix" we make is that we generally consider K, L, M and N numbers "regular" stamps, too. Look around on the forums for an explanation.
The Scott catalogs, used extensively here in the US, are both a curse and a blessing to us First Issues collectors. They provide little help, but plenty of necessity for research.
As always, there's only ONE person who can make the final decision as to how to collect:
YOU 