Long term stability reference

Hi, dear colleagues!

Could You clear next question - 

my customer want series reference  with long term drift about (no worse than) 8 ppm/1kH (1000 hours),

and in hermetic package. 

The same was an LT1021-7 (in TO-92 package), but now it is obsolete.

What actual or pre-release reference do You advice?

Thanks!

Sincerely,

Kirill