Tootfinder

Data to view after voting, see above post
I found lots of websites (¹ ² ³ ⁴) claiming the correct answer is "specced" and only two (Merriam-Webster and Dictionary.com) that referred to the punctuated "spec'd", naming it as an alternative to "specced" with the same pronunciation. (Dictionary.com also named "specking" as a second alternative. I found nothing referring solely to "spec'd".)
However, Google Books Ngram Viewer has "spec'd" as the most common printed form until recently. Continuing the trend past 2019, "specced" should now have a narrow lead. In British English books (not shown), it overtook "spec'd" in 1997 and began a commanding lead in 2010.
Google web searches put "specced" at 3.01M results and "spec'd" at 8.69M, though almost all of the hits refer to "Spec-D" headlights. This is one major advantage of the Ngram Viewer, which doesn't trip over differing punctuation types (it shows "spec-d" at a trickle). It might be more fair to compare "specced out" (348k) with "spec'd out" (739k)