In March 1915 the War Office issued an Army Order regarding soldiers paybook and the wills contained therein.

"In the case of soldiers who have been or may be discharged, or at the conclusion of active service, wills in the possession of officers in charge of records will be sent to the men themselves.

On a soldier giving up his paybook, either after the conclusion of a term of active service, or on the book becoming full, any will contained therein will be cut out and handed to him for disposal as he may think fit.

If a soldier dies before his paybook has been given up, any will contained therein will be cut out and forwarded to the War Office, the paybook being sent to the fixed centre paymaster compiling the man's account."

Therefore many of the soldiers paybook wills belonging to the dead survive.

The majority of those to Scottish Regiments being held at the National Records For Scotland and hosted for viewing (for a fee per mans record) on the Scotlands People website.

Others exists yet in service records, or in the Irish paybook wills collection and the England and Wales collection.

Regarding those held in Edinburgh for those who died in service with the Black Watch.

1,886 are in the collection, to the various battalions of the regiment. (1,965 are now listed. When i did this analysis there were still some formatting issues with the indexing)

For each battalion there exist:

1st: 342

2nd: 244

4th: 73

5th: 23

4th/5th: 152

6th: 194

7th: 222

8th: 272

9th: 239

10th: 23

13th: 26

14th: 58

UK deaths: 6 - (2) 1st Bn, (1) 2nd Bn, (3) 3rd Bn, (1) 2nd/7th Bn.

1,187 have a known grave.

699 are named on a memorial to the missing.

During the war 8,689 men were recorded as dying while serving with the Black Watch.
Therefore the remaining 1,886 soldiers wills represent only 21.6% of the fatalities.

Many wills have been lost, along with their owner in no man's land, or taken by the enemy when captured. Many will have went up in smoke at Arnside, when so many valuable military records were destroyed.
Some will of course exist in the other two collections previously mentioned.

The details they contain can vary.
It depends if they are the basic paybook will, with an empty page with "Will" at the top, which most are, or if they are a "Shortform Will", "Informal Will" or "Formal Will" these have formated "fill in the blank" structure.

The basic Will ones are usually no more than a brief hand written note leaving all their property and effects to a spouse or parent, with a name and address, then signed and dated, giving their name, rank and number.
The others have  much the same details, sometimes with date they were reported killed or missing.

The index to 26,000 soldiers Wills is freely viewable on Scotlands People.