[...] looking at the original plans for the Warre hive it shows the bars only 15/16" (24mm) wide and spaced at just under 1/2" (12mm) rather than the accepted bee space of 3/8" (9.5mm).
Any ideas on the disparity?
As I understand it, your query is regarding the beespace, rather than the comb spacing itself ?
The 'bee-space' is a space large enough so that bees can freely pass through, yet not sufficiently large so that a chain may be formed by two or more bees by which to build comb. So - in practice the beespace will vary, depending upon where in the hive it is positioned.
Vertical spacing is the most demanding. In particular - when considering a conventional framed hive - that spacing between top bars and adjacent bottom bars, and between top bars and crown board (inner cover). This needs to be fairly 'tight', between 6 and 8mm.
Horizontal spacing is far less demanding. For example, the space between frame side bars and the hive walls can be 10mm or so without causing brace comb to be built there. But if this same spacing were to be used for a vertical 'beespace', then comb would most certainly be built to bridge that gap.
Ok - returning to the Kenyan Top Bar Hive and Warre Hive disparity - the difference between them is that the KTBH has bars which are butted firmly together, whereas the Warre Hive has bars which are separated by a horizontal space. As I've just explained, a horizontal space is less critical than a vertical space, and 12mm actually provides (just) 2 x beespaces. Two beespaces are usually found between brood combs, where the bees can work both faces of the combs simultaneously, working back-to-back - so that horizontal spacing is perfectly acceptable to bees.
If I were building another Warre hive, then I'd be inclined to make the Top Bars 22mm wide, providing the slightly larger 14mm (2 x bee-space) gap which the standard self-spacing Hoffman frame provides.
Hope this helps,
LJ