You Screwed Me Again, Pennypacker!

Other than the aforementioned Vandelay, there were a few other pseudonyms that appeared across multiple episodes:

Kramer as "Dr. Van Nostrand"

(The Slicer - 1997)
Kruger: How long have you been doing this Dr. Van Nostrand?
Kramer: Uuh, long long time. Yes, I've seen moles so big they have their own moles. Freckles that cover two men.

(The Strike - 1997)
Krueger: (recognizing Kramer from "The Slicer") Dr.. Van Nostrand?
Kramer: Uh.. that's right.

Jerry as "Kel Varnson"

(The Boyfriend - 1 - 1992)
Jerry: Vandaley Industries, Kel Varnsen speaking. May we help you? (6 year gap between mentions in this episode and The Puerto Rican Day).

Kramer as "H.E. Pennypacker"

(The Millenimum - 1997)
Kramer: Hi, I'm H.E. Pennypacker. I'm a wealthy American industrialist uh, looking to open a silver mine in the mountains of Peru and uh, before I invest millions in a lucrative mine, I, I'd like to go a little native. Uh, Get the feel of their condiments, of their unmentionables, you know, the real uh, gritty-gritty.

Elaine: You screwed me again, Pennypacker!

(The Puerto Rican Day - 1998)
Sales Woman: Did the broker send you over?
Kramer: Uh, yes, most likely, yes. I'm, uh, H.E. Pennypacker. I'm a wealthy industrialist and philanthropist and, uh, a bicyclist. And, um, yes, I'm looking for a place where I can settle down with my, uh, peculiar habits, and, uh, the women that I frequent with. (sniffing wall) Mmm. Mombassa, hmm?
Sales Woman: The asking price is $1.5 million.
Kramer: Oh, I spend that much on after shave. Yes, I buy and sell men like myself every day. Now, I assume that there's a waterfall grotto?

All three come together

(The Puerto Rican Day - 1998)
Sales Woman: Mr. Pennypacker, this is Mr. Vandelay, And you know Mr. Varnsen.
Kramer: Uh, Varnsen.
Jerry: Pennypacker.
Kramer: Vandelay.
George: Pennypacker. Varnsen.

Note - there was a six year gap between Varnsen references. Impressive.