I'm in the middle of some work which includes touching the 856s in lots of
MARC records pointing to websites we control. The websites are available on
both https://example.org/ and http://example.org/

Can I put //example.org/ in the MARC or is this contrary to the standard?

Note that there is a separate question about whether various software
systems support this, but that's entirely secondary to the question of the
standard.

cheers
stuart
--
...let us be heard from red core to black sky