Hans:
Perplexed by the question of what was and wasn't a Contina II or IIa, I recently researched this. I found several book references where an opinion was offered, but the most sensible answers came from looking at original instruction manuals for the II and the IIa.
These highlight the differences are that the IIa has:
• small slots in meter cell flap,
• dual range meter with scoop cut from outer meter dial, which reveals a scale (green for flap closed, and black for flap open),
• EV number scale in meter window,
• lens mount front plate has very shallow horizontal grooves, and
• smaller viewfinder window with no frame around.
The II has:
• no slots in meter cell flap (its just a cover),
• single range meter,
• no scale in meter window because its a match-needle system rather than a pointer,
• lens mount front plate is smooth, and
• larger viewfinder window with surrounding frame/trim.
Apparently, the IIa was produced between 1954 and 1956, and had the "a" in its name to differentiate it from the earlier folding (524/24) camera also called the Contina II.
The II was produced between 1956 and 1958 with some logic to Zeiss dropping the "a" from its name, because production of the 524/24 folding Contina II had ceased in 1953.
I was happy with all this until I found the "Snabb" Contina manual ... which was the Swedish version of the camera (can't remember whether it was a II or a IIa). I decided not to look any further !!!!!
John its not nit picking with Zeiss strange numbering systems and the 'a' can make a difference ie Contax II and Contax IIa. Hans and a great deal of internet reading will give you an idea of the bizarre numbering system of their TLR models.
My Contina has the slot in the meter flap - so its a IIa?