Minor bestseller in Japan for the mid-end segment of the record player market.
A mid-end which here allowed a high-end feature : to upgrade the reproduced sound with the help of the ubiquitous SME 3009 tonearm. A marketing slant rarely used for this not-too-pricey segement but for the later Trio/Kenwood KD-500. Other tonearms were mountable of course but 3009s were the ones most often used on 121s.
The base of the PD-121, however, is built as a one-piece mold so it'll be difficult to tweak the driving part of it.

No sonic masterpiece, no PD-444 or PD-555 but a good... mid-end turntable with glorious looks which welcomes upper-end tonearms and is good enough for plenty of tonearm/cartridge custom matching.
And, yes - this was Micro Seiki engineered.

...here is a PD-121 with another kind of snaky SME,
and a PD-121 with the usual non-snake SME.