In the case of Sinfonias I, II, III and IV, op.73a to 73d (M.194, 196, 200 and 215), some explanation is required as to why you might see either these titles or Kairos, op.73 and be talking about the same music. Well, almost the same...
The first three sinfonias are all single-movement works for strings. The fourth sinfonia, written a few years later, is a work in four movements titled Preludio, Interludio I, Interludio II and Postludio.
Each of the works is capable of performance on its own, and indeed this is the way they were initially performed. However, Holmboe also directed that the sinfonias could be combined into a single large work, Kairos, by playing them in the following sequence:
Preludio - Sinfonia I - Interludio I - Sinfonia II - Interludio II - Sinfonia III - Postludio
There are two recordings available of the sinfonias/Kairos, each of which uses a different strategy to present the compound nature of the work.
The first recording is by Hannu Koivula and the Danish Radio Sinfonietta, made in 1997 but not released until 2004 (Da
Capo 8.226017-18). This 2-disc release presents the sinfonias sequentially on one disc and Kairos on the second (note that the spelling in the image is 'Chairos', even though Da Capo's website now uses 'Kairos').
Sinfonia I from this recording also appears on the Da Capo compilation of its Holmboe recordings, The Key Masterpieces (Da Capo 8.226101-02).
The second recording is by Owain Arwel Hughes and Camerata Wales, made in 2005 but not released until 2009 (BIS CD 1596). This is a single disc release which has Kairos, followed by Sinfonia IV in whole form.
On both of these releases, judicious track selection will enable you to hear any one of the five possible works.