The Halco "Universal Head" was manufactured by Halco Products (Ajax Engineering & Manufacturing Company) of 14230 Birwood Avenue, Detroit, Michigan, USA.
It was designed to attach to the overarm of a horizontal miller and could be supplied with a boss bored to the customer's specification. On early versions, a 1/3 hp motor drove the No. 7 Brown & Sharpe taper spindle through a range of 4 speeds from either 500 to 2900 rpm, or 350 to 2400 rpm; alternatively, at extra cost, an 8-speed model was available. Later models still used the same size of Westinghouse motor, but appear to have been fitted with a choice of 5 or 10-speed drives; the 5-speed unit spanning 350 to 2900 rpm and the 10-speed a very useful 125 to 2,900 rpm.
The quill, with 4 inches of travel, was operated only through a fine-feed control by a 0.75" diameter feed screw, with a 10 tpi Acme thread, running through a bronze nut - there was no quick-action rack feed available. The whole unit weighed 90 lbs.

A picture that shows why the Halco and similar products were so popular - they immediately converted a simple horizontal miller into a machine tool of great versatility and could, providing the operator was content to take delicate cuts, be manoeuvred into the most unlikely of positions. It was also possible to bring the unit into use after a workpiece had been machined on the horizontal miller, enabling a job to be finished, or progressed, without the time-wasting and profit absorbing exercise of dismounting, remounting and setting up on another machine.

Pictures from a 1942 Halco catalog showing the extreme ease of setting up the unit ,and the unlikely positions into which it could be set.

"Tree"  vertical attachment Model MH-4 for horizontal millers by the Tree Tool and Die Works, 1600 Junction Avenue, Racine, Wisconsin, USA.
The unit, like the Halco, could be both swivelled and "nodded", though in the case of the Tree this was achieved by a neatly engineered housing which socketed onto the hole normally occupied by the round overarm of a horizontal miller; a clearer picture of this fitting can be seen here. The unit had a collet capacity of 1/2 inch, 4 inches of quill travel operated by a quick-action rack (there was no fine feed) and 8 all V-belt-driven speeds from either 140 to 3500 rpm or 210 to 5200 rpm. The spindle and quill were hardened and ground and fitted with a micrometer feed stop.
A later version, the MH-4R was fitted with a variable-speed drive and "backgear" with a more useful range of 60 to 3300 rpm.These heads were also fitted to the company's own range of Bridgeport-like vertical millers.

Although marketed under the Kearney & Trecker brand name the useful little Midgetmill and Speedmill units were both made by the Dalrae Tools Co. of Syracuse, New York and described as adaptable to fit "any milling machine". The whole assembly could be rotated about its mounting point - normally the round overarm then common on horizontal millers - and (in the case of the Midgetmill) used for boring and drilling with its fine and quick-feed quill.

Kearney & Trecker (Dalrae) Speedmill was similar in layout to the Midgetmill, but without a quill feed and able to run at up to 5,300 rpm.
More details of these units can be found here.

The Tree head came complete with a swivel housing designed to socket into the housing on a horizontal miller normally used by a round overarm. The design of the housing allowed to head to nod as well as swivel.

The two-speed "countershaft" drive of the Tree head is shown in diagram form on the speed plate.