T206 stamp?

[xplainer]

Not sure if this is a stamp or what.
To me, it appears to be under the blue ink of the card.
Can anyone shed any light on what this is?

[Jobu]

Pretty certain it is an owner's stamp. The left appears to be an ornate D (?) and the right an ornate J.

[Rascal1010]

I have a Mordecai Brown Sweet Cap. Portrait with this exact stamp. I would post a scan, but I sent it in with a group of cards to get graded earlier this week and can not find where I have a scan of the back. It is a very interesting stamp, and I assumed it was an owners stamp, but when I saw this on Jimmy's - Tinker card I questioned him to see if he could shed some light on this stamp. Hopefully someone on this sight has seen it before and can shed some light on this most interesting stamp.

[Bocabirdman]

For some reason that stamp ...uh...that card looks familiar....

I agree. It is an Owner's Stamp......"D J"

[swarmee]

Could be a "P" instead of "D".

[Bocabirdman]

I originally thought that "P" was a possibility but that would put the tail below the bottom of the "J" making it a lower case "P". For initials I presume both to be Upper Case....

[xplainer]

It is a D. After my research, I have concluded it is from the David Janssen Collection and is quite rare.

Oh, and Rat Man.......