Although most examples were built by Delta Rockwell, it appears that these millers were originally made by Harwell Manufacturing of Ft.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 as the "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 sold 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 (the factory may have been located near Sylvania and 36th. Street). Post 1965, production was transferred to a plant in Tupelo, Mississippi, where the last examples were to be made - though the date of their expiry is unknown.
Identical in every detail below the head and ram to the later Delta types, the Centex has parts that are interchangeable in the base, column, knee and table - the only major difference being the drive system. Naturally, Rockwell Delta made some improvements, notably introducing a claver fine-feed mechanism on the left side of the spindle and a worm-gear mechanism for angling the head. The belt drive system was moved to the front of the head and the top speed increased almost four-fold to over 6000 r.p.m.
First appearing in Delta Rockwell catalogues of the early 1960, the early version was available as a vertical-only machine, the Model 21-100, intended for light-duty professional and amateur use. By the mid 1960s it was also being offered, with the addition of a suitable spindle, overarm and a modified drive system (and very few other changes) as a horizontal miller, the Model 21-120 and then, with both horizontal and vertical capacity, as the Model 21-122.  Some machines, badged as the Model 21-820, were fitted with a sophisticated infinitely-variable speed power table drive that featured both jog and rapid approach controls; this unit also sold for fitment to Bridgeport, U.S. Burke, Millrite and Clausing millers.
With an individual Model Number of 21-816, the vertical head fitted to all versions contained a well-supported, six-spline R8 spindle, running on five precision ball bearings within a 3-inch diameter quill with 2.5" of  travel. Feed could be controlled by either a lever-action, rack-operated quick feed or through a worm-and-wheel fine feed with an un-calibrated dial; a depth stop, which incorporated a ruler and micrometer, was fitted as standard. The head and its supporting cylindrical ram were cast in one piece, an arrangement that guaranteed a rigid structure - if not one proof against a tendency to twist in its circular mounting. The entire head assembly could be rotated through 90 degrees in each direction from vertical with the tilt controlled not by hand but a sensitive worm-and-gear mechanism built into the back part of the rear clamp. When the head had been moved through ninety degrees to horizontal, the motor and belt guard were cleverly arranged so that they could be swung around to prevent them from interfering with the access to the workpiece.
Single and three-phase 0.5 and 0.75 hp NEMA C flange-fitting motors were both offered: the 0.5 hp versions of both types could be specified as either 1725 or 1140 rpm, while the 0.75 hp single-phase unit was available as a 1725 rpm model only - and the three-phase version supplied exclusively as a two-speed 1425/1725 rpm unit. With the higher-speed motor fitted the six standard spindle speeds were 370,  700,  1170,  2440,  4420 and a usefully-high 6300 rpm; with the slower motor the speeds became 245,  470,  780,  1620,  2940 and 4200 rpm. The two 5-step V-pulleys were made from cast iron - to promote a flywheel effect - and dynamically balanced to aid smooth running; the motor was arranged to rotate in its housing to allow the belt tension to be slackened when a change of speed was required. A combined spindle lock and brake was fitted, operated by a lever immediately beneath the belt-guard cover on the left-hand side of the head.
24" x 6.5" and with 3 T-slots, the table had longitudinal, traverse and vertical travels of 16.5", 6.75" and 16.5" respectively. Handwheels (in detachable support housings which doubled as coolant and chip collectors) were fitted to both ends of the table; the inclusion of two handwheels must have added considerably to the cost of manufacture - but the convenience would have been appreciated by every owner working against the clock, or faced with a job where, for example, attention had to be paid to alternative sides of the cutter in quick succession.
Internally ribbed and cross-braced, the cast-iron, box-form, knee had its elevation screw (and crank arm) supported on ball-bearings to take the considerable thrust of a heavy workpiece - and lighten the load on the operator as he wound the table upwards; ball races were also used in the collars through which each of the table feed-screws passed. Anther feature that added to the quality of the machine was the use of tapered gib strips to adjust the free play in the table, cross-feed and knee slides.
The miller stood 73.5 inches high, was 37.5 inches wide and 33.75 inches front to back. The cabinet base was 17.5 inches wide and 26.5 inches deep front to back - and the shipping weight approximately 780 lbs..