Backup instruments didn't really become a thing until the band had completed their first album and was supposed to be(come) a professional band. During the club days in 1973 they each had one guitar, period. But when they (hopefully) had to play several nights a week in far off places it was paramount that the show would go on. Instrument malfunctions, should they happen, would not be allowed to derail the performances. Thus Gene ended up with a Fender Precision which can sometimes be seen standing on his side of the stage if the stage was large enough and photographers shot a certain angle.
This bass, which appears top be a completely stock Fender Precision, first shows up during the rehearsals for the New Years show at Academy of Music and when they play their first showcase as a Casablanca recording artist at Filmore East on January 8, 1974, it can be seen standing on Gene's side of the stage.
For a couple of months that's all the backup P-bass does. It stands at the ready on Gene's side of the stage, space permitting, should anything happen to the custom LoBue. Below are a few photos showing this. You can also see it briefly during the ABC In Concert broadcast right as they go into Nothin' To Lose.
In a perfect world the P-bass would never have been called upon to fulfill its role as backup, but alas, in mid-October it had to step in. During the show at the Thunder Chicken in Comstock Park, Michigan, the LoBue apparently developed some kind of issue. Thanks to a spectacular set of photos courtesy Chris Haner we can actually see it happen, how J.R. Smalling comes up on the side of the very small stage and prepares the P-bass for use, and how Gene plays the P-bass—the maple neck and the headstock is a dead giveaway—while Smalling takes up position at the side of the stage with the LoBue right next to him.
It's remarkable, borderline unbelievable, that we actually have a set of photos that show this happening. Whatever the issue the LoBue developed—and later photos of the bass seems to suggest it had something to do with the pickup—for a few shows around this time the P-bass is the only bass caught by photographers. Thankfully this period was brief and the LoBue was soon back in action and the P-bass was once again relegated to standing by the side of the stage (when possible).
As far as we know the P-bass was only called into "active duty" on more time, in Chicago, Illinois. After that it shows up in one more photo from Saginaw, Michigan, as seen below, but not long after the band started getting their endorsement guitars from Gibson and Gene probably decided that he only needed his trusted LoBue and the newly acquired Gibson Grabber.