Hi Villager - I have long been intrigued by this too (my brother got a Springy Sound - which I played a lot - back in the early 80's), the finish seems too thin & flexible to be polyester, yet I was sure it isn't nitro like on my '64 Strat (or '81 Love Rock LS-150).
Turns out it's not nitrocellulose, but it's not polyester either ...
- it's polyurethane!
My friends Greg, Bob & I now have almost two dozen 1978 - 1981 Love Rocks & Springys (& one Breezy! - must fix that) between us. We all use the same luthier, Simon Pinder (Clapton, Stones, Bowie etc. etc. - now based in Hong Kong but worked in Japan for several years, & so of course very familiar with Tokai)
Simon found that the lower models - LS-50s & 60s, and also the ST-50s & 60s - have polyurethane finishes (all except one '78 Springy ST 60 which is nitro).
He says that whilst not as good or easy to patch up, work with or repair as nitro (in fact almost impossible, like with polyester), polyurethane is nevertheless way better than polyester, in that it moves with the guitar with the seasons etc, & can be much thinner than the dreaded polyester - which is a '2-pack' finish (often "instant cured" by UV light nowadays) that sets hard like glass - and can crack the same way with seasonal changes in the guitar. Simon explained that the more flexible polyurethane - when applied thinly and expertly like Tokai do - does not impede the tone like polyester can.
Can't offer any definitive information on which models got polyurethane and which got nitrocellulose in any given year, but I hope this is of some use!