On the other side, this item is marked " PRECISION GAGE & TOOL CO. 28 VOLKENAND AVE., DAYTON, OHIO 45410.   What is it for?  Dimensions are: 3.62 inches long, 1.42 inches wide (36 mm) and .239 inches (7.16 mm) thick.

latest info - this is probably part of an ink gauge, see http://www.pgtgage.com/grindgag.html
(thanks Al Mirman)

This item was described as a "height gauge" - that doesn't make sense to me - what is it?  the steel block is marked T-26240, the top of the casting is marked FC211C.  The 3 bushings you can see in the top are all marked 17-64 (but they measure 0.292 inches ID, which is about .030 larger than 17/64).  Maybe it's some kind of specialty drilling fixture? That seems a bit odd since it is definitely a production item.  It is  7 inches tall as shown in the picture, the two steel shafts are .725 inches in diameter, the steel block is 2.50 inches wide, 1.6 inches high, and 2.0 inches deep.  There are two aligment pins sticking out the bottom of the top casting that align with the pins in the steel block.

From the data below, it's a diode of some kind, but for what purpose and used in what devices:
 

Supplier Data Response for NSN  5960-00-226-3319
Item Name: ELECTRON TUBE

CAGE Code: 79500    Status: A    Type: A    CAO: S3911A         ADP: S3910A

Company Name and Address: WESTINGHOUSE ELECTRIC CORP
11 STANWIX ST                                                  Phone:  412-256-1000
PITTSBURGH PA  15222-1312       UNITED STATES
RPLM Code:                     ASSOC Code: 79500               AFFIL Code:
SIZE: E      Primary Business: J      Type of Business: N      Women Owned: N
SIC Codes:

Management Data Response for NSN  5960-00-226-3319
Item Name: ELECTRON TUBE
PMI: U  ADP:    CC: X  DML: D  DI:    ESDC:    HMIC: P
ENAC:

Characteristics Data Response for NSN  5960-00-226-3319
Item Name: ELECTRON TUBE
 MRC   Requirement Statement          Clear Text Reply
NAME   ITEM NAME                      ELECTRON TUBE                      
AWGH   TUBE TYPE                      DIODE                              
Reference Number Data Response for NSN  5960-00-226-3319
Item Name: ELECTRON TUBE
TYPE II: M    INC: 00001    HCC:

                                           I       D
                                           S RN RN A RN
 Part Number                      CAGE  SC C CC VC C AAC          SADC

 575R721H01                       79500 A  2  5 2  X SB
 575R721H01                       97942 A  2  3 2  X SB

Thanks to Mark, I have some additional information on this part, it looks like it was in use as recently as 1991 in something, but what?

The part number is in our database (rev F dated 1966-02-09, rev G dated 1991-12-20 most current entry).  There are no users indicated in the on-line database no current need for this part.  The source control drawing is owned by Test Design Engineering (group within NGC-ES, in Baltimore, that "owns" the part). There may be paper archives where one could find out where this part had been used. 

updated 03/07/2007