Do you know what year the Pfaff made a machine with the numbers 138-6121-BS

You probably need the serial number:

Pfaff Questions Answered When was my Pfaff Sewing Machine Made

not sure if this will help you out.PFAFF was founded in Kaiserslautern, Germany in 1862 by instrument maker Georg Michael Pfaff (1823-1893). Pfaff's first machine was handmade, and designed to sew leather in the manufacture of shoes. In 1885, Georg Michael Pfaff opened a sewing machine shop in London. The PFAFF factory was expanded and modernized...
or go to this link. what year was my pfaff sewing machine made Google Search

Pfaff has many of there manuals here: http://www.pfaff.com/en-US/Support-and-Updates/Support . I found a manual for the 138 using the search option after clicking on the down arrow in the Machine Name box. The direct link to the manual is http://www.pfaff.com/SiteMedia/PFAFF/Products/Machines/Support-manuals/Pfaff_138-139-manual-ES-DE-FR-ES.PDF . However, the only place I see any reference to 6/21 is in the section on needles. (Some 138 models used a specific needle system.) I don't see any letter codes after the 6/21 in the manual.

I hope this helps.

Cindy Wells

You might try joining some Yahoo Groups that are dedicated to older Pfaff sewing machines. There are a couple called:
Pfaff_fixit
and
oldpfaffpforum
whose members are very knowledgeable.

...

Contact manufacturer and explain

Support site with phone number
http://www.pfaff.com/en-US/Support-and-Updates/Support

Machine Support

I can only find manuals up to 91 and 130 and up. Perhaps you can look at the pictures on the manuals and see which one looks most like yours. Pfaff manuals are very difficult to find.
http://www.pfaffmanuals.com/

Not all machine serial numbers have production information built into them. Better off finding out when the model was made based on a model number not a serial number.

Cheers,
Kelly
San Diego, CA
Embroidery Services

Try this site:
http://www.ismacs.net/pfaff/pfaff_manufacture_dates.html

Your serial number appears to be from 1899.

On the label on the back of the machine where the serial # is, in the lower left corner is the 1/4 and the year that is was made. Example (IV/01) Between August and December of 2001. The IV are Roman Numerals.