I’ve done some research on what shocks are available for the Husqvarna Svartpilen and Vitpilen 401. I figured I'd make a copy of the links and share my notes
Many shock manufacturers don’t list our bikes, but list the very similar KTM 390, so I included that in my search.
Especially for piggyback reservoir shocks, note that the many changes with the 2020+ Huskies (especially the exhaust routing) may effect what fits on the bike. And I think for a piggyback shock to fit at all, it will have to be installed “upside down” with the piggyback reservoir on the swingarm end. I don’t know if that is a problem, but it would certainly be subjected to a lot more vibration and movement there. And it would add a bit to the unsprung weight.
And of course we cannot assume the KTM 390 shocks will fit either, that need to be verified.
I think my personal preference is for a shock without a separate reservoir, as that simplifies installation. I might be convinced to try a shock with a separate “hose attached” reservoir, but I wouldnt want to clamp it it too prominent of a location.
Options listed below I've provided more detail on the shocks I was personally more interested in.
Ohlins
KT 301 : 300mm long, rebound only
KT 801 : 304 +10/-2 mm long, external reservoir, comp and rebound
KT 303 : 300 +9/-3 mm long, external reservoir, comp and rebound - for RC390
STX46 Street Performance Line is based on our popular STX46 DR1 shock absorber and developed for naked sports bikes and street performance bikes....
www.ohlins.com
The Öhlins STX 46 shock absorber comes in a wide range of styles and for many different bikes in the smaller Hyper Sport segment.
www.ohlins.com
KT 301:
https://www.ohlins.com/product-item/54059/
KT 801:
https://www.ohlins.com/product-item/63079/
KT 303:
https://www.ohlins.com/product-item/54981/
https://www.ohlins.eu/en/products/motorcycle/kt-301--4773/
https://www.ohlins.eu/en/products/motorcycle/kt-801--6614/
https://www.ohlins.eu/en/products/motorcycle/kt-303--5211/
Ohlins Pricing:
https://store.hardracing.com/ohlins-shocks/ktm-ohlins-shocks/
https://mnnthbx.com/product/ohlins-kt303-for-ktm-390/
Nitron
https://www.nitronusa.com/buy-online/motorcycles/husqvarna/401-svartpilen/-18-20-
https://mnnthbx.com/product/nitron-ntr-r1-2017-husqvarna-vitpilen-401/
NTR R1 : adjustable preload, height and “combined rebound compression”
YSS
https://www.yss.co.th/racing_product.php
Shock Factory
https://shock-factory.co.uk/522-shock-absorber-ktm-390-duke-13-18
Wilbers
https://www.wilbers.de/shop/en/Motorcycle/KTM/390-Duke-KTM-IS-Duke/?cur=5&year=2016
JRi
https://www.motorcyclistonline.com/long-term-ktm-rc390-suspension-upgrade-jri-shock-installation/
Penske
https://www.penskeshocks.com/product/8900-series-2015-ktm-rc390/
Hyperpro
https://www.webshop-hyperpro.com/en...uspension/rear-suspension/rear-shock/HU04-0AA
https://www.webshop-hyperpro.com/en...uspension/rear-suspension/rear-shock/KT03-0AB
https://www.webshop-hyperpro.com/en...uspension/rear-suspension/rear-shock/KT03-1AB
RacingBros Shicane HLR Edge
https://www.steadygarage.com/store/racingbros-shicane-hlr-edge-rear-shock-for-ktm-duke-390-rc390
Matris
https://www.matrisdampers.com/wp-content/uploads/2017/04/KTM-RC-390-15-16-web.pdf
EMC
https://www.emc-suspensions.fr/en/3077-390-duke-2018-2020
K-Tech
https://www.ktechsuspension.com/road/
Yacugar
https://www.teknikmotorsport.com/?rf=kw&kw=husqvarna+401+shock
Bitubo
https://www.bitubo.com/Bitubo/cms/ITA/category-cms/31-road-racing.html