Identifying someone with a really large DNA match

[LizzieL]

Should be easy shouldn't it?
I tested with Ancestry some time ago and uploaded my raw DNA to as many sites as I could, one being MyHeritage
Every so often they email me to say I have matches, but today I got a message to say I have a close match. I looked, and the match is 665.7 cM across 21 segments, the longest being 64.1 cM.

My biggest on Ancestry is 546 cM across 16 segments, Longest segment: 115 cM. (My first cousin's daughter)

Even though Ancestry does a little pruning and is not quite comparable, my new MH match is pretty close, but I cannot identify him at all.
He has a tree with only himself in it giving his birth year. He has a surname which doesn't appear anywhere in my tree -
He's in his 20s and I'm in my 70s, so that excludes a lot of relationships. I have no children, I have one sibling who has no children, so that excludes great nephews.
I can see from shared matches that he would be related to my paternal grandfather's mother's line, so that excludes the only first cousins who could have a child in their 20s - they're on maternal side.

I can find two possible births which fit his year of birth, one has a mmn the same as his and location is better fit. The other has a different mmn, but I cannot find a marriage for parents. Possibly his father married a widow or divorcee so her surname at marriage is different from her maiden surname.
Can't find him on Electoral register, probably chose not to be on public register
So Facebook is my last resort I think!

[Gadget]

Hi Lizzie

I won't go through the results/investigative pathways, as you seem to have done a thorough examination, have you tried to contact him?

Gadget

[LizzieL]

Not yet, I wanted the challenge  of trying to identify him through the usual on-line methods first. But if I continue to draw a blank I will message him.
It just seems such a close match, I should know him. I know all my first cousins and their children and  many of my second cousins, particularly on my father's side. Although most I haven't seen since I was a child. Despite the large cM match, I wonder if I should look further than 2nd cousins.

[Spelk]

Hi Lizzie,
If your sibling is male then he might have had a child he knows nothing about. However that would also give matches on your mother’s side and the cM are a bit low for a great nephew.
Might be you have a half sibling you do not know of.
Certainly you should try to get in touch but you may have to wait. I had a similar unknown high match and it took six months before he replied. After that the truth came out within a few days.

[Nanna52]

I was very excited to find a match of over 300cM on ancestry linked to my unknown biological grandfather.  Many attempts to work out the link were met with silence.  Eventually I dug around and started a tree for her having found out her parents.  Dug a bit more she had the same great grandparents as I, on one side, on the other they were her grandparents.  Toss that one out I can’t work out that.
Another good match I found on a different line had the same 4 X great grandparents as me.  Too large a match.  We compared out trees and found that three of our direct ancestors had siblings who had married into each others lines.  Small area and population.
Nothing is easy with this hobby!

[Gadget]

The only matches that I have that is close to that are 1st cousins.

Painter gives these possible relationships:

Add - what are the closest shared matches?

[LizzieL]

Yes, I saw that and can eliminate everything in the 56% category, because of age and shared matches

Closest shared matches on MH are

H who shares  94.1 cM with me (predicted Parent's 2nd cousin, 3rd cousin) and shares 34.6‎ cM with X ( predicted Grandparent's 4th cousin, Parent's 4th cousin)

G who shares  80.8 cM with me (Predicted 3rd cousin) and shares 35.2‎ cM with X (predicted Parent's 3rd cousin, Parent's 4th cousin)

H is actually my 3rd cousin (his great grandmother was the sister of my great grandmother) G is actually my  3rd cousin once removed) His 2 x great grandmother was another sister of my same great grandmother).
Then there are matches I can't identify then:
K who shares  42.7 cM with me (Predicted 4th cousin) and shares 23.7‎ cM with X (predicted Parent's 4th cousin)

K is my half third cousin once removed (her 2 x great grandmother was the half sister of my same great grandmother).

So X and I must share my 2 x great grandfather as there are connections to descendants of both his wives. But we must connect much further down the line as the match is so high.

[Gadget]

Have you used the Chromosome Browser tool -

Chromosome Browser
A tool for viewing shared DNA segments between you and multiple DNA Matches, which can help point to a common ancestor.

I've had a few successes  with it.

[LizzieL]

I downloaded the matching segments and pasted them on to DNA painter, There were so many segments it's a real mess now.
My main marker for that particular 2 x great grandfather is a huge section on C19. X, H and K all match here, but strangely not G. There also know descendants of his from Gedmatch in that region too.
Unfortunately my closest relative who shares that same 2 x gg-father is only on Ancestry
He's a 2C1R sharing 107 cM across 4 segments, much lower than my new match.