Do early JB strats have a "swimming pool" route as Strat Plus had? Are they made of alder? What are the differences in specs between those early JB strats and Strat Plus?

Thanks in advance for your answers,
Peter

Both guitars used the same bathtub rout.

The JB signature model is a souped-up version of the Strat Plus with some special features favored by "El Becko" himself, such a huge baseball bat neck and a humbucking Gold Sensor Dually in the bridge with a push-push button for single or dual-coil pickup operation.

Both the JB and the Plus sported a standard tone control for the neck pickup and a TBX circuit for the middle/bridge pickups, with the latter having a Hipshot Tremsetter. These guitars were also equipped with a Wilkinson roller nut and Sperzel tuners.

As of 1993, all Plus, Deluxe Plus, Ultra and Jeff Beck series models featured an LSR roller nut, Schaller tuners and an early 1950s spaghetti headstock decal.

Here's a pic of my Jeff Beck strat body, with the routing exposed. Mine was made in 2000.

All of the "modern" Fender Strats had the swimming pool route from their introduction (first with the American Standard in late 1986) until the American Deluxe line came out in 1998. That would cover these models:

Amercian Standard Strat
Amercian Standard Strat Deluxe (short lived model with gold Lace Sensors but no roller nut/locking tuners)
Strat Plus
Strat Plus Deluxe
Strat Ultra
Jeff Beck Signature Strat
Lone Star Stratocaster
Roadhouse Stratocaster
Big Apple Stratocaster

One of the "upgrades" (or "updates" if you prefer) for the American Deluxe Strat was the move to the HSH routing (like heustis's Beck Strat). The American Deluxes first came out in 1998. At that point the Strat Plus/Plus Deluxe/Ultra were discontinued. During 1998 the other modes (American Standard, Jeff Beck, Lone Star, Roadhouse, and Big Apple) were transitioned from the swimming pool route to the HSH route. I'm sure Fender had stockpiles of pool routed bodies in certain colors; for example, once the pool routed white bodies were used up in 1998 the next batch was done with the HSH routing. You will find pool routed bodies in some colors with later dates in 1998 than HSH routed bodies in other colors due to this transition.

Not sure about, but some FSR models came with an HHH rout instead of the HSH. Also, a very few Japanese-made Contemporary and HRR Stratocasters were routed for HHH.

The big advantage of the HHH rout is the possibility to throw a third humbucker in the middle position. The tonal possibilities of HHH pickups combined with a super 5-way pickup selector and the S-1 switch are almost endless.