Together the centers provide reproductions of several hundred outstanding brasses, the special paper and wax required, and instruction to beginners. Most facsimiles are reduced by cold casting or computerized photoengraving to convenient size.

Westminster Abbey, which attracts three million tourists a year, has the largest and busiest center. It offers a selection of 104 replicas from 12 by 24 inches to 30 by 72 inches, though only a few from the abbey itself. While the two other London centers stock fewer brasses - 70 at St. Martin's-in-the-Fields and 25 at All Hallows - they are less crowded, so attendants can give customers more personal help. For those who don't care to make their own, collections of fine rubbings may be seen in the British Museum, Victoria and Albert Museum, Society of Antiquaries, all in London; the Cambridge Museum of Archeology and, at Oxford, the Ashmolean Museum and Bodleian Library.

It seemed to us that anyone can make a rubbing worthy of hanging on the wall on first try after brief instruction. Observing amateurs from 4 to 90 years old at the three London centers, we saw no failures.

For each brass there is a leaflet giving the date, location and history of the original. In medieval and Tudor times, the engraver made no attempt to portray the individual memorialized. The figures are typically stylized and depicted in costume of the day - nobles in armor, ladies in kirtle and headdress, bishops in mitre and tunicle.

Our selection, for example, showed the five sons and three daughters of Master and Dame St. John as identical in age, size, pose and long, fur-trimmed gown.

''Their father, Oliver,'' the leaflet told us, ''was a justice of the peace in Lincolnshire from 1472 and sheriff in 1489. His sons continued to widen the family influence; a great-grandson was created Viscount Bolingbroke in 1611.''

Westminster supplies only black paper and waxes in gold, silver and copper. St. Martin's has white as well as black paper and, in addition to the same waxes, more in rose, white, blue and green.