Re: Einstein/Roosevelt letter.

This famous letter from Einstein to Roosevelt,

was written on Aug.2 1939, prior to the start of WW2 which is regarded
as Sept.1 1939, so it is somewhat out of the bounds
of this group, "soc . history . war . world-war-ii" and is also very
scientific, so that part lacks most historians interest.

But the letter was delivered on Oct.11 1939, in WW2, and FDR's
reply was on Oct.19 1939.
Note in FDR's reply, where he thanks Einstein quote,
"and the most interesting and important enclosure".
Note the word "and".
Enclosure is NOT the same as disclosure, as would be
found in the body of the covering letter, but is an addendum

Einstein and TEAM had already surveyed the worlds Uranium
supplies, determined the feasibility of a fission and a fusion
weapon (Teller), garnered intel on German research and then
went on to suggest, quote,
"However, such bombs might very well prove to be too heavy for
transportation by air."
Which was absolutely true at that time. That implies the TEAM
had already worked out an approximation of the A-bombs
weight and size, thus providing the specs for a future atomic
bomber, the B-29, and had hardened up a tactical requirement.

Can anyone provide the contents of the "enclosure"?
Is it still classified?
Ken S. Tucker