existing foreign key that references fields with different datatype (char(8) and varchar(8))
Digging into long forgotten boxes I found both nt4 and SQL-Server 6.5 installation media and I can confirm that a foreign key referencing fields with different data types was allowed.

Here is the statement I successfully run on SQL-Server 6.5:

create table tbl1 (code char(8) primary key (code))
create table tbl2 (code varchar(8) primary key (code))
alter table tbl1 add constraint FK foreign key (code) references tbl2 (code)


no trailing semicolon or you'll get a syntax error.

I can't get a grasp on sql 7 media so I can't tell if it was working on that version too.