Scholars say this represents a wooden frame with a sliding movable「tongue」extending from one side, used to make measurements similar to a caliper…significantly, a CALIPER is generally used to MEASURE GIRTH, DIAMETER of a shaft, since straight-edged measuring tools can best measure straight edges. A tool that「repeatedly wilts, shrinks」is one that also repeatedly stretches and enlarges, or else it could not shrink again. The 'jaw' of a caliper opens wider or narrows in order to perform measurements.
And quite obviously, used to MEASURE GIRTH, DIAMETER of a shaft by means of「tonguing that woody」with plenty of experience for comparison. VARIABLE SIZE AND SHAPE definitely applies to any situation in which「skilled tonguing with woody」occurs.
Other explanations are also plausible considering both the elements as boneless body parts, a FRAME OF MEASUREMENT.「Dildo on orifice」used to determine the correct size to fit one's orifices. In dynastic China, female masturbation, particularly in the harems, was a glorified activity without any shame involved. Naturally, as a member either aged and/or was stretched significantly, her two orifices would require larger sized tools in order fully satisfy urges and desires, hence the need for a SIZING FRAME when crafting individual pieces, a task that was performed with great pride. A「woody tongue」that progressively widened could ascertain appropriate orifice girth.
「Historians do not know who invented the dildo but most believe that dildos most likely originated in Asia. Dildos have been found in China that date back as early as the New Stone Age, that's more than 12,000 years ago and most recently a tomb was uncovered from the Han Dynasty in Xian, China that was home to a rather interesting collection of dildos. Seven cast bronze dildos were discovered and evidence indicates that they were used by palace maids on sexually deprived imperial concubines or aristocratic women during the Han Dynasty. Also discovered in ancient China, are handcrafted dildos which double as works of art…」
[ link to text and images is now dead, unsurprisingly, but was https://chinahub.blogspot.jp/2009/02/ancient-chinese-dildo.html 2012-09-11 try your own Internet search instead ]…no shame there, it seems. Note that PORCELAIN literally means「vulva of a sow, female pig (see: obese concubine 豕).」
CHINESE JUNIPER「…is a juniper that grows as a shrub or tree with a very variable shape, reaching 1–20 m tall.」
[ https://en.wikipedia.org/wiki/Juniperus_chinensis 2016-02-10 ]

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: Juniperus chinensis; measuring-frame
Japanese EDRDG: builder's frame for measuring
Unihan extended: a builder's frame for measuring