Although most examples were built by Delta Rockwell, it appears that the Centex milling machine was made originally by Harwell Manufacturing of Ft. Worth, a company in the ownership of one Red Harwell. First produced circa 1958 to 1959, the machines were initially branded "Centex" and designed to compete with the similar-sized Clausing 8500 Series. One distinct advantage for the Centex was its use of Bridgeport-type R8 tooling, rather than the popular but probably less effective No. 7 Brown and Sharpe employed by Clausing. It's likely, but not yet confirmed, that Mr. Harwell was able to sell his design to Delta-Rockwell in 1960, and for the next five years the line continued to be made in Ft. Worth as the Delta Rockwell (the factory may have been located near Sylvania and 36th. Street). After 1965 production, was transferred to a plant in Tupelo, Mississippi, where the last examples were to be made - though when production ended is not known.
Very well constructed, and identical in every detail below the head and ram to the later Delta Rockwell  types, all the feed shafts were supported in ball bearings - the three handwheels, the knee elevation shaft (at both ends) and the vertical knee screw. All fasteners were either socket-head or cross-slot screws with a complete absence of hexagon-headed bolts anywhere on the machine itself. Unfortunately the table was set unusually high - at its lowest setting around a foot above that on the larger Powermatic Millrite for example - and for the smaller operator this made for an uncomfortable reach. 
Naturally, upon acquiring the design, Rockwell Delta made some improvements: a completely altered drive system, the fitting of a clever, two-rate fine-feed mechanism on the left side of the spindle and a worm-gear mechanism for angling the head. With the modified drive (it was moved to the front of the head) one of the speed-range options gave a top speed of just over 6000 r.p.m. , a rate almost four-times that of the previous highest available; although likely to be seldom-used, this was still a handy specification to have on any small vertical miller.
More details and additional photographs of the Centex will follow. If any reader has an example, or literature concerning the machines, the writer will be pleased to hear from you.
With thanks to Rex Burkheimer for the excellent photographs.