Weird but I definitely have seen this on Roland/Ibanez amps, maybe it's a Japanese thing.
As in "44+44V" (obviously DC) rails being fed from "44+44 V" transformers , in the parts list or other schematic versions they were identified as "35V " transformers, end consensus was that in **some** amps Japanese refer to transformers (which they probably don't even design but buy from giant OEM suppliers) by "the voltage (DC of course) you will get by using that transformer"
I found no other explanation.
If design work is very compartamentalized, split in many areas not communicating at low level, amp designer *might* spec: "my amp needs 42+42V rails at 2.5A DC , please source 1000 transformers for them within 30 days"
Not as weird as it seems for us Electroguys, a car designer might order "an axle of such and such dimensions, such tensile strength, such Brinnell surface hardness, such polishing and tolerance" and neither specify nor care about steel composition, tempering, annealing, etc. , just that it meets or exceeds his goals.
So I read this transformer as:
* P14-P15: 400*.707=283VAC (290V?) winding @ as many mA needed for this 30W amp.
I bet 200-250mA would be acceptable, anything above a bonus.
* P16-P17 47*.707=34VAC . 35-36VAC would be fine too.
Any current rating is fine, almost 0 consumption there.
* P18-P19 and P19-P20 those are never converted to DC so "they are always 3.15+3.15VAC" . Not very consistent, huh?
* P21-P22 ... now the plot thickens ... considerably.
This "13VAC" winding is rectified through a bridge and is regulated by a "7810".
So in theory we have 10VDC there and everybody is happy.
*Problem* : those 10V DC power *both* V1 and V2 filaments, each in series so needing 12.6V DC *and* the TS circuit requiring nominal 9V ... so 10V DC is wrong on both counts.
Maybe the designer had a brainfart and thought that "splitting the difference" was a good idea?
Not to me.
I would wind P21-P22 to, say, 13VAC as shown , get some raw 17/18VDC (would also increase C20 to 1000uF or even 2200uF if it fits) , use a 7812 regulator for true 12VDC filament voltage (well within spec) and increase value of R46 from original 110r (what's used in the TS pedal version) to 1k or 1k2, which should drop voltage to about 9V DC .
This winding should be specified for some 400-500mA.