Just got an email from Deep Cert.
He confirms that this is 66L5.
Here is a reference copy:

"There is that heavy dot off orn H as discussed. Unfortunately the left margin dots halfway down (as shown in Neinken) are under cancel.
In addition, there are some fine scratches shown in the Neinken mat at orn L/M, they don't scan well, but they do match with the posted stamp.
Siegel sale 1006 lot 1266 is a strip, 66-67-68L5, ex Neinken, which interested parties could download as reference. No doubt Neinken drew his mat using this strip."