I unearthed my 1967 original copy bought by me at time of release, and I have the mono version. Odd, because from 1966 on I always specified stereo copies of albums if they were available.
....I know I own this album, bought it when it came out, in 1967, but I think I own the stereo version, which should have 588086 as the catalogue number.
Later pressings when Polydor lost the licence to press and distribute Atlantic product in the UK would have had a 'K' prefix, although, there appear to be no copies of Kinney / WEA / WB UK reissues on the discogs listing for this album (31 variants in all).
I might be wrong, and the album may have been deleted. The 1973 West German reissue has cat.# ATL 40 013, so K 40013 would be a likely candidate for a UK reissue catalogue number. ....ATL 40 013 now linked...
A note on the UK stereo issue at Discogs, for 588086: "According to the Vanilla Fudge website this one has a different mix from the US issue and is noted for it's richer sound and deeper bass levels."