Detectives are hoping an expert in Germany will provide clues about the gun

It is a small handgun, produced in Germany between 1922 and 1930, but police are hoping the specialist can date it more precisely.

It is more than five months since Mr Wilson was shot dead on his doorstep.

The investigation by Northern Constabulary has yet to establish a motive for the killing.

It happened in November as Mr Wilson was preparing his two young sons for bed.

Detectives revealed that the semi-automatic 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.

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

Investigators 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.

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.