...that the line you are looking at is FORD's way of allowing TV pressure being set on the CRUISE-O-MATIC, similiar to the procesdure to adjust the TV (throttle valve) rod/cable on the later AOD trans.
The CRUISE-O-MATIC did not receive an vacuum actuated modulator valve until the 1961 COM redesign. Prior to this model change, the dual range COM and the single range FORD-O-MATIC (and MERC-LINK variations) had the actual shifting controlled by actuating linkage coming off the throttle bellcrank. The linkage adjustment of this design included monitoring pump (line) pressure, and I am assuming this fitting was placed here on the BIRD as the chassis design made most service sequences tight.
Alexander has thoughtfully supplied a scanned copy of The FORD MECHANIC booklet (these were distributed to techs of the period to introduce them to new product and service requirements) and this issue details the 1958 BIRD.
On this page you will notice service procedure for the COM. At the end of the section, it says;
For complete information on CRUISE-O-MATIC service procedures, refer to "The 1958 FORD MECHANIC No.2"
These publications are more informative in this regard as it is accepted that those that later use the shop manuals are more trained/progressed in repair and most theory is not necessary to present at this point.
I know I have this publication and I am sure it is where I saw the illustration of the fitting. Once I find it, I will post it here.