Dave Hollins sent me the following additional info:

IR63 are included in Geschichte der K. und K. Wehrmacht by A. Wrede Vol2 (1898) https://books.google.co.uk/books/about/Geschichte_der_K_und_K_Wehrmacht.html?id=-58PAQAAIAAJ

One of the last flags of the 5 Walloon regiments is discussed in: 'A propos du drapeau du régiment d'infanterie Murray n° 55' in: 'Carnet de la Fourragère' Sept. 1962, pp. 369–376 (also in Archivum Heraldicum (a Swiss magazine)). IR55 Murray carried an Ordinärfahne with the red diagonal Cross of Burgundy behind the Doppeladler and the arms of St. Omer below the eagle. However, IR58 Vierset (until 1794)/Beaulieu (1794–98) had a regulation 1780/92 Ordinärfahne with the bar through 'J' to create 'F'. Laid up in Würzburg, this flag was destroyed during WW2, but a photograph survives in the Bildarchiv of the Austrian National Library in Vienna.

While helping Terry Crowdy with his Marengo book (USA: September https://www.casematepublishers.com/marengo.html - UK end June 2018 - pls use Hive https://www.hive.co.uk/Product/Terry-Crowdy/Marengo/21922284 ) with the benefit of a memoir by Hauptmann Schellenberg of 1st Battalion IR23, I had another look at IR63. They secured the bridging point for the Laufbrucke north of Marengo, but they also took very heavy casualties in the battle and one source says they finished up at La Barbotta farm, a bit further north. Schellenberg describes being wounded in an assault on Marengo hamlet, but Rauch of 3rd battalion IR23 just describes marching through. The casualty figures for Latterman's Grenadiers are consistent with the last stage of the battle, but not with an assault on Marengo, which is the usual story. So, as IR63 had their Grenadier companies with them, the probable course of events is that IR63 launched the first assault om Marengo and after being driven out by Rivaud, they attacked again, reinforced with 1st Battalion IR23.