Back in 1922 the A492 was created as a shortish route from the A44 at Llangurig to Caersws, where it met the A489. Cutting off the corner in Llangurig was the A4085, which became a spur (and possibly even the mainline) of the A492 in 1935.
For some reason, as part of the 1935 renumberings the route was extended along the A489 into Newtown, where it terminated on the A483, leading to a useless multiplex as the A489 continued at either end.
This anomaly was rectified more recently with the extension of the A470 route to provide a complete North-South route across Wales. At that point, the original section of the A492 became part of the new route, and its extension reverted to be the A489 once more, meaning that the A492 number became defunct.
In fact the A470 south of Caersws does deviate from the line of the ex-A492. Llanidloes is now bypassed (with the old road now the B4518). Secondly, the original A492 in Llangurig itself is now closed to traffic and the A470 uses the more southerly route of the road (the original A4085).