Two Chicago tournaments of games that had been needed to be sorted out at one point for the year.
The remaining 6 of 18 games (and maybe as many as 1 or 2 others to sort out: Biographer Bistro thanks for replying...) look like they are the Chicago Invitational, and not the USA open, 1973 that 2 or more Canadians (Vranesic from U of Toronto wouldn't have been too far from the windy city, Suttles from UBC would've had more of a distance to travel...) may have attended that year. I have no idea whether one or both players would've played both events, unless this <Invitational> was also held as some sort of qualifier that year for some continental event or perhaps an Olympiad selection within Canadian ratings lists, etc.
Held at Chicago in the end of November (28th) until December (9th, according to David Moody's Chess Life and Review link posted in a spring 1974 issue (unchecked yet be me as of Nov 9, 2017) - page 157, March).
1 Norman Weinstein 8.0/11
2 Gudmundur Sigurjonsson 7.5/11
3 Nikola Karaklajic 7.0/11
4 James Tarjan 7.0/11
5 Andrew Soltis 5.5/11
6 Kim Commons 5.5/11
7 Zvonko Vransevic 5.5/11
8 Craig Chellstorp 5.0/11
9 Greg DeFotis 5.0/11
10 Lawrence Kaufman 4.0/11
11 Andrew Karklins 3.5/11
12 Duncan Suttles 3.5/11
In contrast to:
(1) J Manetta vs Suttles, 1973 5th round, month of August - according to its pgn - https://chesstempo.com/gamedb/game/... also has a similar pgn;
(2) Suttles vs B Diesen, 1973 4th round, month of August - according to its pgn;
(3) G S DeFotis vs L Day, 1973 10th round, month of August - according to its pgn;
(4) Suttles vs Lombardy, 1973 10th round, month of August - according to its pgn;
(5) E B Meyer vs B Diesen, 1973 9th round, no month marked - according to its pgn;
(6) Tarjan vs Suttles, 1973 may also now look like Chicago Invitational, according to its pgn: round 8; December.
Game Collection: US Open 1973, Chicago
1973 US Open: Chicago Illinois, August 12-24
Held in Chicago around the middle of August.
Round 1: Sunday, August 12
Round 2: Monday, August 13
Round 3: Tuesday, August 14
Round 4: Wednesday, August 15
Round 5: Thursday, August 16
Round 6: Friday, August 17
FREE DAY August 18
Round 7: Sunday, August 19
Round 8: Monday, August 20
Round 9: Tuesday, August 21
Round 10: Wednesday, August 22
Round 11: Thursday, August 23
Round 12: Friday, August 24