A=440 means the concert pitch (2nd B on tenor ,low F# on alto ,2nd F# on bari ,low B on soprano and 1st A on C melody ) all sound as 440Hz and if you go up an octave it would be 880Hz. Low pitch on vintage horns means 440 Hz and high pitch is above this so they cannot be used in modern music. A standard guitar tuner with a microphone for tuning acoustic guitars can be used for tuning a saxophone but all horns from around 1926 were low pitch except a few made by conn and possibly hawkes and son which went to low pitch by around 1930 Dave

If you want a true A=440, get an A tuning fork. I use that one and a C=523.25 too. They are also a good demonsrtration for kids learning about pitch. Electronic tuners are great but not absolutely true. Sometimes I have to tune my synths as high as A=444. A=440 is the A above middle C on the piano. C=523.25 is the C above that.