The metal tube covers are RFI/EMI shields for tubes in sensitive position like phono stage etc.
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Re 6550c's question, a great many metal jacketed tubes were produced, way back when. They were widely used in radios and tv's, and some audio equipment, from the mid-1930's until the end of the tube era in the late 1960's. They were probably most popular during the late 30's and the WWII era. Besides emi/rfi protection, the metal envelope supposedly resulted in improved ruggedization.
Many tube types were produced in both metal versions and glass versions.
Although some types of metal tubes contained a glass tube within the metal envelope, most did not.
Following is a summary of some historical information that is provided in the book "Tube Lore," written by Ludwell Sibley, a noted authority on early tubes and president of the Tube Collectors Association.
The first metal tubes were introduced by RCA and GE in April 1934, and used a steel shell, with individual glass-filled eyelets for the lead-in wires.
Some competitive manufacturers, such as Arcturus and Triad, then introduced a few types that had a conventional glass tube within a metal envelope. These had the suffix "MG" (for "metal-glass") at the end of the type number, such as 5Z4MG, 6J5MG, and about 15 or so other types, most of which were introduced in 1935 or 1936. That approach quickly fell out of favor, however, and very few MG types were subsequently introduced.
Most manufacturers, including RCA, eventually moved to a "button stem" construction, in which the lead-in wires were routed through a glass disk, which was sealed into the bottom of the tube's metal envelope. That approach was apparently first introduced by Arcturus.
Few if any metal tube types were introduced after WWII.
Re the op's question, as has been said the tube shields provide rfi/emi shielding. I have some experience operating vintage equipment on which the tube shields were missing, and I've never had any noticeable symptoms that resulted. But I would certainly suggest leaving them in place, given that the equipment was designed with them. Besides the shielding they provide, it seems conceivable to me that removing them might have significant effects on both the operating temperature of the tube and on stray capacitance.