Hello, I'm doing research into my grandfathers service in the RAF during world war 2 and am looking for advice.

I've located the only serviceman with his name in the AIR78 index. (Thanks to this site)
The index card is stamped with a capital F inside a circle which I understand means this person signed up pre-war sometime between 1935 - 1937, approx

The catch is that the service number is too low. It is allocated for 'Oct 1925 civillians'
If this is the correct person then it means he signed up when he was 7 years old.

Here is what I need help with

1. Could he have signed up late 1930's and still be allocated a service number from the 1925 range?

2. Is it possible for class F reserves to be posted over-seas? I though this was a no-no
I have photos of him as an NCO wearing what appears to be warm weather uniform ie. light colour.
The photo frame is original and from a photographers studio in Adelaide, Australia.

3. On another airmens index card I've seen the letters 'KS' appended.
Does anyone know what these letters mean?

4. Without being sure of the service number, I'm planning to apply for my grandfathers service records without stating a service number.
Could this cause a problem?

Thanks in advance