[Boatanchors] more sp-600 questions

[Norman J McSweyn]

Hi gang!
I've got a few more questions!
1: The various military manuals refer to a serial number for parts identification as well as troubleshooting. The question is to which serial number does it refer. My radio has two serial numbers. One is on the badge plate on the front panel which says it is a r-542/frr sn 390.Doing a little detective work revealed that it really is a r 274c/frr. 
The plate on tuning unit assembly cover states that it is a jx-14 sn 7703. 
Which one do I use?
2: T3 (if transformer, crystal filter) has a piece of masking tape on it bearing the words 456 xtal (sic) installed. The rest of the sub assembly seems to be original. Does anyone know why this xtal was changed from 455kc? I do assume that this (value) was done with a purpose in mind, rather than that value was on sale at the local electronics store that day. 
Orange drops come Monday. I can go from the non-destructive phase to the uh-oh I can't believe I just did that where in the hell am I gonna get one of those phase.
tnx, 
n3ykf