Peter's point is well taken. Though it is my understanding that the certificate was in aiki-bo, Ueshiba also taught Hikitsuchi a set of bokken forms apparently drawn from Shinkage Ryu called "Sho-Chiku-Bai."
One also needs to remember that a number of Ueshiba's students had periods of military service in occupied lands at the periphery of the Japanese Empire in the Thirties and Forties -- during the Thirties, he also taught at the Nakano Gakko, the Toyama Gakko, other naval and army training centers for officers and non-commissioned officers, and Kenkoku University in occupied Manchuria -- and some of those students became intimately acquainted with the use of a sword on living flesh, though one wonders how often the victim was, in fact, armed.
More interesting is the provenance of the "Iwama bokken."
When the Founder was alive, his wife Hatsu mended his keiko-gi and hakama until they were threadbare. The bokken he used in Iwama was the handle for a hoe, and his jo the handle of a water ladle for watering crops in the fields. I hope that these instructors can remember that most likely when they arrived in the United States, the uniform they owned was worn, and the bokken and jo they carried were not expensive and probably carried in a hand-made case.
In the United States it seems at times like we are kindergarten students dressed up in adult professional football gear. We get lost in the materialistic trappings and forget the true spirit of Aikido. Football is played all over the world…in most countries however it is played barefoot with only a ball…
The situation with koryu is quite different. Individual koryu use the sword in differing ways and each has its own preference with regard to weight, thickness, curvature, blade & tsuka length, and so forth. Without access to a qualified instructor, one can easily misconstrue the pedagogical intention or combative application of the choice of bokken for regular training, training exercises or paired forms publicly demonstrated.
This site has a nice set of links to images of bokken used in various schools.
(I've never handled bokken from this supplier, but people whose judgement I trust have, and they have been complimentary about the quality of workmanship).
Inasmuch as they have trained in koryu systems, whether formally or informally, many aikido instructors use a bokken of the type associated with whichever koryu they studied.
Hope this helps,