If header is shorter than 64 bytes, it comes from a version that does not support dumping some of the structures. It should then be interpreted that the data for missing structures could not be provided, not that the size of that data is zero.

The newer version of this documentation is intended to be more complete and corrected and I wouldn't really want you to use the older one.

The thing is: this document should a base for writing a tools that should work with all .fas files, no matter what version of fasm produced them.

You do not really believe this, don't you ?

The same I wanted to have with .fas - I had to devise my own format only because none of the existing debug info format were capable of containing all the data fasm can provide and which can be useful for various purposes.

In your example with the obj file - well it is not important which version (or which program) created it - every linker will accept it. BUT the object file definition does not change from version to version.
(didn't proove - just assume it)

As for the old versions of fasm, you can find them on the old website.

irp j,ja,jb,jc,je,jg,jl,jo,jp,js,jz,jae,jbe,jge,jle,jmp,jna,jnb,jnc,jne,jng,jnl,jno,jnp,jns,jnz,jpe,jpo {
        macro j args \{
                match byte, short \\{
                        j args
                \\}
        \}
}
irp v,  Ah,Bh,Ch,Dh,Eh,Fh,\
        A0h,A1h,A2h,A3h,A4h,A5h,A6h,A7h,A8h,A9h,AAh,ABh,ACh,ADh,AEh,AFh,\
        B0h,B1h,B2h,B3h,B4h,B5h,B6h,B7h,B8h,B9h,BAh,BBh,BCh,BDh,BEh,BFh,\
        C0h,C1h,C2h,C3h,C4h,C5h,C6h,C7h,C8h,C9h,CAh,CBh,CCh,CDh,CEh,CFh,\
        D0h,D1h,D2h,D3h,D4h,D5h,D6h,D7h,D8h,D9h,DAh,DBh,DCh,DDh,DEh,DFh,\
        E0h,E1h,E2h,E3h,E4h,E5h,E6h,E7h,E8h,E9h,EAh,EBh,ECh,EDh,EEh,EFh,\
        F0h,F1h,F2h,F3h,F4h,F5h,F6h,F7h,F8h,F9h,FAh,FBh,FCh,FDh,FEh,FFh,\
        AF0Fh,BA0Fh,D9DEh,E0DFh,FFFFh,FFFFFh,FFFFF0h { v equ 0#v }
include 'fasm.asm'    

ln -s expressi.inc source/expressions.inc
ln -s preproce.inc source/preprocessor.inc
ln -s assemble.inc source/assembler.inc    

129c129
<      14c:     2e 0f 01 16 88 02       lgdtw  cs:0x288
---
>      14c:     2e 66 0f 01 16 89 02    lgdtd  cs:0x289
190c190
<      1fc:     68 ff ff                push   0xffff
---
>      1fd:     6a ff                   push   0xffff
969c969
<      a21:     66 69 c9 0a 00 00 00    imul   ecx,ecx,0xa
---
>      a21:     66 6b c9 0a             imul   ecx,ecx,0xa
1928c1928
<     16db:     0f 84 7e 00             je     0x175d
---
>     16d8:     74 7e                   je     0x1758
7852c7852
<     5924:     0f 84 7f 00             je     0x59a7
---
>     591f:     74 7f                   je     0x59a0    

system.inc:
;...
        lgdt    pword [cs:GDTR]         ; load GDT register
;...
GDTR dw 2*8-1                           ; limit of GDT
     dd ?                               ; linear address of GDT
;...    

Someone starting from zero with fasm still needs flatassembler.net, or another site, hosting the original zips.