By tournament:

1932 Hong Kong & British Navy Sevens
1934 Blarney Stone Sevens
1963 Bill Riach Sevens
1976 Hong Kong Sevens

The list may be subject to change if more rugby union sevens tournaments are discovered.

Hong Kong & British Navy Sevens

An article appeared in the Hong Kong Daily Express of 29 February 1932, regarding a sevens tournament organised by H.M. S. Tamar.

Preliminary rounds were played on 9 March with the latter rounds and final being held on 12 March.

H.M.S. Medway were runners-up in a seven-a-side Rugby Football Tournament, in which several Naval teams competed. The results were:— First round.—Hermes 11, Club B 0; Medway A 6, Kowloon B 0; Kowloon A 21, Tamar B 0: Medway B 6, Bank 3; Club A 26, Tamar C 0. Second round.—Cumberland 9, Tamar A 3; Medway A 8, Hermes 0; Medway B 8, Kowloon A 5; Club A 6, South Wales 13orderers 5. _ _ Somi-finals.—Medway A 8; Cumberland 0; Club A 13; Medway B 0. Final.—Club A 8, Medway A 0 . The Naval teams comprised.—Cumberland A: Mid. Thurstan, L. S. A. Squire, I,ieut. Fisher, R.M., Lieut. Gallimore, Paymaster-Lieut. Kennett (capt), Surgeon- Lieut. Brosnan. Hermes A: Lieut. Wilkinson, A.B. Rumpleman, Sub.-Lieut. Martin, Flight- Lieut. Colquhoun, Lieut. Huhback (capt.), E.R.A. Doggett, Lieut. Padfield. Tamar and small ships A: Tel. Martin, Surgmn-Lieut. Kempthorne, Mile. Wells, E.A.R. Lestherbr, Lieut. Watson. Sub.- Lieut. Paine, Cpl. Ainsworth. R.M. Medway A: I.ient. Woods, Sto. Packer, A.B. Northwood. Lieut. Ryder, Lieut.- Commander Crick (capt), Lieut. Linton, Lieut. Pizev. Medway B: Ord. Art. Fab_rey, Ja E.R.A. Kerr, Lieut. Price Ldg. Cook mes, Plumber Spring (capt.), Sig. Knight, E.R.A. Lomas.

https://www.britishnewspaperarchive.co.uk/viewer/bl/0001973/19320408/242/0013

Blarney Stone Sevens

The Blarney Stone was a building in Hong Kong. It seems it was used as a mess for single employees in Hong Kong; and those Blarney Stone residents started a Sevens tournament. The teams played for the Blarney Stone Shield.

A report from the Hong Kong Daily Press of 11 March 1935 states that the holders, the Hong Kong and Shanghai Bank team, were knocked out.

The tournament looks to run between 1934 and 1941, stopping for the Second World War, and then from 1947 to 1986.

Bill Riach Sevens

The Bill Riach Sevens tournament was named after a serving Hong Kong policeman who died in 1963. The Hong Kong police website states:

Not many officers still in the Force would remember Bill, who served from 1957 until his untimely death in 1963. By all accounts he was a well-liked and respected officer who was Canadian by birth but always claimed Scottish ancestry. His lifestyle mirrored his rugby exploits in that he was an aggressive, slightly unorthodox scrum half who played a committed but fair game. At the time of his death he was travelling back to Sha Tau Kok Station, where he was the SDI, when he failed to negotiate a bend and ran off the road into a tree.

As a measure of the popularity of the man, the Bill Riach Sevens was instituted later that year and have been played on an annual basis ever since.

Hong Kong Sevens

An international Sevens tournament is somewhat out-of-scope for this site. However the Hong Kong Sevens has developed into one of the largest Sevens tournaments in the world. It began in 1976.

From ‘The Official History of the Melrose Sevens’ by Walter Allan:

Following the success of the Scotland Centenary Sevens held in Murrayfield in 1973, a group of Hong Kong expatriates created a truly international annual sevens competition and with the sponsorship of Cathay Pacific and Hong Kong Bank, it rapidly became the tournament where the best players in the world take part.