Google search seems to indicate that the SB-240 final was/is an 8873 not two 3-500s.

Sindre Torp wrote:

Hello,

I have a Heathkit SB-240 linear amplifier. This is a prototype!!! It
matches the SB-104 line in design.
I got this one from my friend Mike Elliot (the man behind SB-104 and Drake
TR7). He is now silent key.
The SB-240 has two 3-500Z tubes, and built in power supply. I have full
documentation for it.
I have it stored in USA at the moment. I'm considering selling it for a
serious price. That's why I have not got it shipped over to Norway yet,
where I live.

I don't think there are any more SB-240 out there. I've never heard of any.
If you have any more info about the SB-240, please drop me an e-mail.

You may take a look at it he
http://home.online.no/~sindtorp/sb240front.JPG
http://home.online.no/~sindtorp/sb240rear.JPG

Regards,
Sindre Torp, LA6OP
http://home.online.no/~sindtorp