The boxes are meant to accept Kadee centerset "G" type couplers, plastic lids, and springs from Kadee kits such as the Kadee 789 and Kadee 907 "New Generation" assemblies, evolutionary differences of which are illustrated below. Kim JY, Haastert PV, Devreotes PN (April 1996). "Social senses: G-protein-coupled receptor signaling pathways in Dictyostelium discoideum". Chemistry & Biology. 3 (4): 239–43. doi: 10.1016/S1074-5521(96)90103-9. PMID 8807851. As to mounting the #1 Gauge coupler from the 1907 kit on the GP40, it seems possible that some shortening of the loco's post and the pilot lip that protrudes could be trimmed off to obtain the proper coupler to rail head height. Since my original intent for designing the Datum Precision coupler box employing true Kadee centerset couplers was to avoid doing any cutting on the GP40, this would seem counterproductive. Attwood TK, Findlay JB (February 1994). "Fingerprinting G-protein-coupled receptors". Protein Engineering. 7 (2): 195–203. doi: 10.1093/protein/7.2.195. PMID 8170923.

The picture below is an overall view of the UP GP40 having the #1 scale coupler from the Kadee 1907 in the Datum Precision box mounted on the front of the loco. Though the coupler from the Kadee 1907 kit is the primary focus of this topic, consideration is, also, given to the Kadee 1917 kit's "#1 Scale" centerset coupler that has an extremely short shank.It's intended that the Datum Precision coupler box is to accept a Kadee centerset coupler (e.g. having no up or down offset - coupler from the Kadee 789 or 907 kits) for mounting on the Aristo-Craft GP40 and other applications. The number 912 is very compact, and the ideal gearbox for cars with a short or open platform end, like tank cars or cabooses:

Rosenbaum DM, Cherezov V, Hanson MA, Rasmussen SG, Thian FS, Kobilka TS, etal. (November 2007). "GPCR engineering yields high-resolution structural insights into beta2-adrenergic receptor function". Science. 318 (5854): 1266–73. Bibcode: 2007Sci...318.1266R. doi: 10.1126/science.1150609. PMID 17962519. S2CID 1559802.GPCRs downstream signals have been shown to possibly interact with integrin signals, such as FAK. [48] Integrin signaling will phosphorylate FAK, which can then decrease GPCR G αs activity. In that vignette an example Kadee 880 gauge coupler height measured 1.115 inches from the rail head surface (or 32.33 inches if scaled up to prototype 1/1) - thus, the Kadee gauge is within the prototype allowance with respect to CFR 49, section 232.2 - it being somewhat below the center of the CFR range of 33 inches. The Kadee description for the 907 & 1907 includes the statement ".... # 1907 #1-Scale Centerset Couplers & #911 Gearboxes"