The gun is small enough to fit into the palm of a hand

Detectives have revealed that the pistol was a Schmeisser, of which about 40,000 were originally manufactured.

The gun is small enough to fit into the palm of a hand and police believe it may have been brought into the UK during or after World War II.

Mr Wilson was shot on the doorstep of his home in Nairn last November in what appears to be a motiveless murder.

The pistol was found in a drain about half a mile from Mr Wilson's home where he lived with his wife Veronica and two young sons.

Crimewatch feature

Detectives released the gun's details ahead of the murder being featured for a second time on the BBC's Crimewatch programme on Wednesday.

They said it had been manufactured between 1922 and 1930 by C.G. Haenel Waffen of Suhl, Germany. The pistol was mass-produced and exported throughout Europe.

Measuring just 116mm (approximately four-and-a-half inches) and weighing 390g (13.7oz), it is commonly known as a "pocket" or "waistcoat" pistol and is sometimes referred to as a "ladies'" gun because of its size.

The type of bullet used in the shooting

The weapon has "C.G. Haenel Suhl-Schmeisser Patent" embossed on the side of the barrel and an "HS" monogram on the grips.

Detectives believe it may have been brought into the UK as a "war trophy" by service personnel during or after World War II.

Other explanations are that it was legally imported into the UK prior to 1997 or that it was smuggled into the country.

They also revealed that the ammunition used in the shooting was .25 ACP/6.35mm calibre, manufactured by Sellier and Bellot, Vlasim, Czech Republic.

It may at one time have been held legally in the UK but it too could have been smuggled into the country.

Envelope link

Mr Wilson died after going to the door of his home and speaking to a man.

He returned inside with an A4 envelope, then went back to the front door, where he was shot. The envelope has not been found.

Grampian Police have conducted an independent review into the force's progress in the case, and Northern Constabulary have liaised with a number of outside agencies, including Interpol, in an attempt to find the killer.

Mr Wilson's funeral was held in Fort William last week.

A �10,000 reward has been offered through Crimestoppers for information leading to an arrest and conviction. A national newspaper has offered another �10,000.