I wonder how many there are of these Aerograme/letter sheets for Yugoslavia there are. If there are not too many, I might add them to my Yugoslavia Airs collection.
The two types you have illustrated (Workers with tools and Plane in clouds) are the only two types.
The Workers type exists both as an envelope and a letter sheet:
The letter sheet exists in two shades of brown-red, one considerably darker than the other.
The envelope exists in four forms:
a) The third and fourth red lines for the address are 52mm long
b) The third and fourth red lines for the address are 54mm long and the word Salje (which means Sender) is on the back of the envelope
c)The third and fourth red lines for the address are 54mm long and Salje does not appear on the back.
d) A dark blue V shape on the lower flap at the back.
The Plane type exists in three forms:
a)The third red line for the address is 54mm long
b)The third red line for the address is 49mm long
c)The blue overlay pattern is a bright blue rather than the dull grey-blue of the other two types.