Interestingly, the OED cites a number of potential "rubber" tools for the
plasterer, stone mason and metalsmith.
Not sure which of the following if any apply to your inventory, but it
might give you some leads to pursue.
JL

A metal or stone implement used for rubbing, esp. in order to smooth or
flatten a surface. Now chiefly hist.

1794   D. Steel Elements Rigging & Seamanship 88   Rubber, a small iron
instrument, in a wooden handle, to rub down or flatten the seams.
1835   J. Holman Voy. round World III. xiv. 422   The plaster is..smoothed
with a rubber, until it acquires an even surface.
1850   C. Holtzapffel Turning & Mech. Manip. III. 1089   The Rubber used by
Masons and Statuaries is frequently a slab of grit stone, to which a handle
is attached by means of an iron strap.
1875   E. H. Knight Pract. Dict. Mech. 1997/1   In the moldings of stone,
an iron rubber mounted on a wooden stock is employed for fillets, beads,
and astragals.
1933   S. Casson Technique Early Greek Sculpt. i. i. 36   The hard stones
are worked with abrasive rubbers for the general surfaces and outlines.
2008   V. E. Szabo Monstrous Fishes & Mead-dark Sea v. 152   The most
common objects found on early sites include..maintenance implements
including combs, smoothers, rubbers.

A pad or roll of soft material used for rubbing and polishing, spec. a
piece of wadding wrapped in a cloth and used in the application of French
polish.

1839   A. Ure Dict. Arts 801   The polishing rubbers [for marble] are
coarse linen cloths, or bagging, wedged tight into an iron planing tool.

A large, coarse file, esp. one used for metalwork. In later use chiefly
more fully rubber file.

1678   J. Moxon Mech. Exercises I. i. 14   The Rough or Course Tooth'd File
(which if it be large is called a Rubber).
1771   Invoice 3 Dec. in G. Washington Papers (1993) VIII. 558,   4 Smiths
Rubber Files.
1837   J. Bennett in N. Whittock et al. Compl. Bk. Trades 225   The very
heavy files, such as smiths' rubbers, are made of the inferior marks of
blistered steel.
1846   C. Holtzapffel Turning & Mech. Manip. II. 825   Rubbers..measure
from 12 to 18 inches long,..and are made very convex.
1875   E. H. Knight Pract. Dict. Mech. 1997/2   Rubber-file, a heavy,
fish-bellied file, designated by weight, which varies from four to fifteen
pounds.
1905   Dental Cosmos 47 1001/1   With a rubber file or any flat rough file,
file the solder away outside the pins.
1916   Proc. National Acad. Sci. 2 128   The tools required in metallurgy,
as those for..shaping=97hammer, file, rubbers, polishers.
2000   Modelling & Painting Figures 22 (caption)    Begin the sanding with
a rubber file.
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